Try posting the DDM diagnostic report you have generated. It will reveal why its not working.

Thanks Swamped -- here's the lastest DDM report:

Diagnostic Report, generated 11/30/2018 (10.0.18290)
Copyright (c) 1995-2018, EnTech Taiwan.
Version = 1.40.0.1928
OS = 10.0.18290.2 x64
Mechanism = 3
Account = Standard
Command line = "C:\Program Files (x86)\Dell\Dell Display Manager\ddm.exe" 
////////////////////////////////////////
EnumErrorCount = 0x00000000
LastCheck = 0x00000000
HintShown = 0x00000001
////////////////////////////////////////
11:08:42.102...Log opened
11:08:42.318...LanguageID = 00000409 00000409 00000409
11:08:42.335...Build = 1.40.0.1928
11:08:42.336...Windows handles = 000201A6 00010146 000101AA
11:08:42.337...OS monitor sleep timeout value = 14400 ms.
11:08:42.337...Applied DPI = 96-96
11:08:42.337...VM = 0
11:08:42.644...Starting enumeration
11:08:42.866...Ending enumeration
11:08:42.881...New=DEL40DD&VEN_10DE&DEV_1B80&SUBSYS_33661028.0002 DEL40EA&VEN_10DE&DEV_1B80&SUBSYS_33661028.0001 DEL40EA&VEN_10DE&DEV_1B80&SUBSYS_33661028.0003
11:08:42.919...Video power down timeout = 14400
11:08:42.922...User session connect
11:08:42.927...Display powered ON
11:08:47.955...Refreshing screen object
11:09:01.550...\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
11:09:01.560...Dump: complete Windows device enumeration
11:09:01.566...1.DeviceName      = \\.\DISPLAY1
11:09:01.571.....DeviceFlags     = 00000001
11:09:01.575.....DeviceString    = NVIDIA GeForce GTX 1080
11:09:01.584.....DeviceID        = PCI\VEN_10DE&DEV_1B80&SUBSYS_33661028&REV_A1
11:09:01.589.....DeviceKey       = \Registry\Machine\System\CurrentControlSet\Control\Video\{0D025A1A-F3FF-11E8-AC76-701CE7D64A6C}\0000
11:09:01.599.....a.MonitorName   = \\.\DISPLAY1\Monitor0
11:09:01.603.......MonitorFlags  = 00000003
11:09:01.605.......MonitorString = Dell UP2716D (DP)
11:09:01.606.......MonitorID     = MONITOR\DEL40DD&VEN_10DE&DEV_1B80&SUBSYS_33661028\{4d36e96e-e325-11ce-bfc1-08002be10318}\0002
11:09:01.609.......MonitorKey    = \Registry\Machine\System\CurrentControlSet\Control\Class\{4d36e96e-e325-11ce-bfc1-08002be10318}\0002
11:09:01.614...2.DeviceName      = \\.\DISPLAY2
11:09:01.615.....DeviceFlags     = 00000005
11:09:01.617.....DeviceString    = NVIDIA GeForce GTX 1080
11:09:01.618.....DeviceID        = PCI\VEN_10DE&DEV_1B80&SUBSYS_33661028&REV_A1
11:09:01.619.....DeviceKey       = \Registry\Machine\System\CurrentControlSet\Control\Video\{0D025A1A-F3FF-11E8-AC76-701CE7D64A6C}\0001
11:09:01.623.....a.MonitorName   = \\.\DISPLAY2\Monitor0
11:09:01.625.......MonitorFlags  = 00000003
11:09:01.628.......MonitorString = Dell U2717D (DP)
11:09:01.631.......MonitorID     = MONITOR\DEL40EA&VEN_10DE&DEV_1B80&SUBSYS_33661028\{4d36e96e-e325-11ce-bfc1-08002be10318}\0001
11:09:01.633.......MonitorKey    = \Registry\Machine\System\CurrentControlSet\Control\Class\{4d36e96e-e325-11ce-bfc1-08002be10318}\0001
11:09:01.641...3.DeviceName      = \\.\DISPLAY3
11:09:01.647.....DeviceFlags     = 00000001
11:09:01.650.....DeviceString    = NVIDIA GeForce GTX 1080
11:09:01.654.....DeviceID        = PCI\VEN_10DE&DEV_1B80&SUBSYS_33661028&REV_A1
11:09:01.661.....DeviceKey       = \Registry\Machine\System\CurrentControlSet\Control\Video\{0D025A1A-F3FF-11E8-AC76-701CE7D64A6C}\0002
11:09:01.668.....a.MonitorName   = \\.\DISPLAY3\Monitor0
11:09:01.674.......MonitorFlags  = 00000003
11:09:01.677.......MonitorString = Dell U2717D (DP)
11:09:01.680.......MonitorID     = MONITOR\DEL40EA&VEN_10DE&DEV_1B80&SUBSYS_33661028\{4d36e96e-e325-11ce-bfc1-08002be10318}\0003
11:09:01.685.......MonitorKey    = \Registry\Machine\System\CurrentControlSet\Control\Class\{4d36e96e-e325-11ce-bfc1-08002be10318}\0003
11:09:01.690...4.DeviceName      = \\.\DISPLAY4
11:09:01.692.....DeviceFlags     = 00000000
11:09:01.695.....DeviceString    = NVIDIA GeForce GTX 1080
11:09:01.697.....DeviceID        = PCI\VEN_10DE&DEV_1B80&SUBSYS_33661028&REV_A1
11:09:01.699.....DeviceKey       = \Registry\Machine\System\CurrentControlSet\Control\Video\{0D025A1A-F3FF-11E8-AC76-701CE7D64A6C}\0003
11:09:01.703...5.DeviceName      = \\.\DISPLAY5
11:09:01.706.....DeviceFlags     = 00000000
11:09:01.711.....DeviceString    = Intel(R) HD Graphics 630
11:09:01.715.....DeviceID        = PCI\VEN_8086&DEV_5912&SUBSYS_07DB1028&REV_04
11:09:01.717.....DeviceKey       = \Registry\Machine\System\CurrentControlSet\Control\Video\{A923FD73-F3F3-11E8-8A2E-BDF9907378BB}\0000
11:09:01.721...6.DeviceName      = \\.\DISPLAY6
11:09:01.724.....DeviceFlags     = 00000000
11:09:01.725.....DeviceString    = Intel(R) HD Graphics 630
11:09:01.726.....DeviceID        = PCI\VEN_8086&DEV_5912&SUBSYS_07DB1028&REV_04
11:09:01.727.....DeviceKey       = \Registry\Machine\System\CurrentControlSet\Control\Video\{A923FD73-F3F3-11E8-8A2E-BDF9907378BB}\0001
11:09:01.730...7.DeviceName      = \\.\DISPLAY7
11:09:01.731.....DeviceFlags     = 00000000
11:09:01.731.....DeviceString    = Intel(R) HD Graphics 630
11:09:01.734.....DeviceID        = PCI\VEN_8086&DEV_5912&SUBSYS_07DB1028&REV_04
11:09:01.736.....DeviceKey       = \Registry\Machine\System\CurrentControlSet\Control\Video\{A923FD73-F3F3-11E8-8A2E-BDF9907378BB}\0002
11:09:01.742...Total number of monitors enumerated = 3
11:09:01.743...\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\

- edit dupe post - original post didn't show up for a while

- edit - dupe post

Nothing is being enumerated by Windows, though gfx cards and monitors are obviously present. Its possible a system update left device management in an exceptional state. Try the following steps:

1. Open Device Manager (devmgmt.msc)

2. Uninstall everything *under* "Monitors" (not the Monitors hive itself) by right-clicking on each entry and selecting "Uninstall"

3. Uninstall everything *under* "Display adapters" (not the Display adapters hive itself) - display settings will change but not to worry since they'll be changed back in the next step

4. Now go up to the Action menu in the DevManager menu bar and select "Scan for hardware changes"

Your NVidia display driver should be installed from scratch and the monitors re-registered