Dell PowerEdge R640 stopped booting

This sounds much more like a no POST / board-level fault than a boot-device problem, because you have iDRAC access, PSU LEDs are green, the fans do the brief spin-up, but the server never gets to video or power-button control. The next most likely causes are the system board, a CPU/socket issue, or a shorted add-in/backplane component that is preventing POST.

What to try next

Rule out the power button/front panel path.

Use iDRAC to try a remote power on/off cycle.

If iDRAC can’t assert power either, that points away from the front panel switch and toward a board/power-sequence issue.

Strip it down even further.

Remove all PCIe cards, RAID controller, GPU, extra NICs, USB devices, and all drives.

Leave only:

system board

1 CPU

minimum DIMMs in the correct slots

one PSU

If the machine still won’t POST, the fault is likely in the board, CPU, or memory path.

Try one CPU at a time, if this is a dual-socket config.

Boot with only CPU1 installed, then only CPU2 if needed.

A bad CPU, bent socket pins, or thermal/retention issue can stop POST completely.

Reseat the memory carefully.

Use Dell’s minimum-memory population rules for the R640.

Bad DIMM placement can sometimes look like a dead board with no useful log entry.

Check for a hidden hardware fault using iDRAC/Lifecycle logs.

Even if the main log looks empty, look for power, CPU, memory, or “watchdog” entries around the failure window.

A job queue or firmware issue can also hang startup, though that usually still shows some POST activity.

Test with known-good parts if possible.

Known-good PSU, RAM, and CPU are the fastest way to isolate the failure.

If all minimal configs behave the same, the system board is the prime suspect.

Thanks for the response @Dell-Martin S

Seems it is an issue with the motherboard. I was using a line interactive UPS with the server. When the server is running with the UPS battery, the PSUs rapidly blinks with green. Might it be the case to fail the motherboard?

I have had this exact issue caused by a bad power supply. iDRAC would work, everything would light up, but when asked for power-up... Nothing.

I'd test your server with a known-good power supply before condemning the system board.

@XenoniaN Thanks.

Are you now using an online UPS after the PSU replacement?

@WINSF​ I received the server used and with the issue already present. 

I connected it to my Smart-UPS after PSU replacement and have not had any issues with it. 

The UPS is an APC Smart-UPS X 2200VA in Green Mode.

@XenoniaN , when you are running on the battery on this APC UPS, does the server PSU handle stay solid in green color?

@WINSF​ Yes, steady green, no issues.