Delll optiplex 755 desktop - motherboard is dr845 13740. Problem with display. When boot up, no graphics on monitor - same as if monitor not connected. monitor checked ok but pc shows no visuals?

First try another monitor to be sure that monitor is bad, or the PC motherboard.
Hope this helps.

Do you have a good working monitor that you can troubleshoot with? I would make sure the monitor isn't going bad first. Then if you're still having the same issue, try re-booting in safe mode and run a virus scan.

Press and hold the F2 key while booting. In Bios, return everything to the factory defaults. This should do it.

Troubleshooting guide @ lights

http://support.dell.com/support/edocs/systems/op755/en/UG/HTML/trouble.htm#wp1302403

Looking at the specifications for the Optiplex 755 line I'm seeing the highest power supply is in the minitower version at 305W. I can't find a document specifying the power requirements for the Radeon x1900, but most benchmark power consumption tests show the system pulls around 280 - 300W with that card installed.
The problem is either your specific Optiplex 755 configuration can't support the power requirements, I don't know if you have the MiniTower, Desktop Tower, Small Form Factor or Ultra Small Form Factor version of the sytem or the card is itself is bad.

Either your monitor is bad or the video card inside the computer is bad. To isolate the problem try a different monitor on the computer. Borrow a monitor from a neighbor, friend, relative or co-worker. If the borrowed monitor works, then your original monitor is bad. If the Borrowed monitor does not work, then the video card inside your computer is bad.

If is possible we need to establish if the monitor is defective or not. Please connect to a different monitor/LCD TVscreen or just unplug the monitors data cable from the back of the computer powered it up and check if is giving you a message on the screen as "No signal detected". It will tell you that the monitor is fine and is a problem with your Dell computer.

Do you mean your motherboard optiplex 755 not working?
If you mean it, and you have to do troubleshooting and found that the motherboard does not work, it is clear your motherboard problem.If you want to fix it you must change the motherboard to another (new)Your have to make sure that you change the motherboard that can support all the hardware on your computer, especially processor, ide slots, SATA, display card (if using) and ram your.If you choose a motherboard that can support all existing hardware on your computer, your computer should be fully functional.
I hope the tips provided to be helpful,
thank you

Generally an indication that the Hard Drive isn't detected
Things to check, go to BIOS F2, reset defaults and reboot, see if the HDD is seen. If it is, the battery on the motherboard is likely dead and needs replacement (Watch battery at Walmart, 3 dollars)
If the HDD doesn't detect after resetting the BIOS, then most likely a Hard Drive failure

ple.... check the ram & cables..... after boot in system