755 does not have DVI so thats the first issue.

You cannot have any monitor attached to the VGA port if there is a card in the video slot.

Un supported Video Configuration

Diagnostic LEDs for the OptiPlex series (2005 to 2009)

https://www.dell.com/support/article/us/en/04/sln284978/a-reference-guide-to-the-dell-optiplex-diagnostic-indicators

123    LED with non AMBER power button is POST complete — Onscreen error message

1. 755 MT, DT, SFF, USFF (what PCI-express slots have video GPU card, nvidia quadro nvs290 )
2. what PC is it, of the 4???? with x16 slot its NOT USFF. so down to 3,
3. remove power cord to PC and wall.
4. push power button for 10seconds release, this act discharges all CAPs making system safe (no pc damage)
5. remove old RTC coin cell.
6. install new, not backwards.
7. connect  unstated monitor to VGA if GPU card not present. (it is present i guess)
8. or if GPU card is present connect monitor to your DVI port.  (nvidia quadro nvs290) do not use VGA on main chassis of PC, only used CARD PORTS ONLY. VGA/DVI/HDMI/DP)
9. now power on and hammer the F2 key at dell logo. bam you land in BIOS land.
10. and we pick that BIOS option, called, set bios defaults, to reset.  now back to factory settings.
11. but factory settings will be wrong, for home use, so you in the same bios pages turn OFF PXE boots.
12. last but not least make sure the boot disk (SSD/ or HDD0) is first in the BIOS boot order page.
13. exit and save BIOS and boot to your unstated OS.
14. Do not panic or do rash things, ask for help,  (rash like loading new OS, unstated what by you)

last but not lease, that card is not DVI card. direct

it has  DMs59 cable that is split to 2 DVI ends, do you have that? try both ends?