I got a dell optiplex 755 base unit from work as they were throwing them out but I cannot get a picture on 2 different monitors,no beeps no bios nothing.I connected a key board and mouse (both usb).Unit powers up dvd drive and hard drive sound like they are working,green leds on the power button and green leds on 3 and 4 on front panel.

How to Reset BIOS or Admin Password Dell Optiplex 760 780 745 755

hi There.
this is all i could find out from the internet so not sure if this will help you out.
(I went into BIOs again after I looked the SATA port over on the motherboard. The SATA ports on the motherboard are listed as 0,1,4,&5. In BIOS the SATA ports are 0.1.2.3.4. & 5. I have been trying to turn on 2 or 3 as they are the next ports and never tried 4 or 5.
When I turned on 4 and 5 everything is working fine.
Why do they list SATA ports 2 and 3 when they are not present on the motherboard)
or see if this link helps you.
Dell Optiplex 755 2 sata ports 2 3 not working on motherboard Google Search

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

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.

I don't know if you have done anything lately,ie; updates to board, cahnge of cables, etc. but first, try shutting down system, unplug your cable for video, unplug tour tower, plug tower back in and restart. If this doesn't work. You may want to try a system restore back to when you had a picture. Try this and let me know if this worked by adding comment or reply at the end of solution.
Dave

1. What steps you take to solve a particular beep code will differ depending on the BIOS manufacturer.
   Choose the correct beep code troubleshooting guide below based on yourmotherboard's BIOS manufacturer:
   
   • AMI Beep Code Troubleshooting (AMIBIOS)
     
     
   • Award Beep Code Troubleshooting (AwardBIOS)
     
     
   • Phoenix Beep Code Troubleshooting (PhoenixBIOS)
   Tip: Not sure what brand of BIOS is on your motherboard? See this How to Determine Your Motherboard BIOS Manufacturer guide for some helpful advice.
   

Just had similar problem with optiplex 755. Solved problem by resetting bios. Remove power cord. remove bios reset jumper. (check owners manual or dell website for the location) Dell says to remove for 5 seconds and replace to reset bios. I find this to be incorrect. I had to unplug cord remove jumper, plug cord in, restart, shut down, unplug cord replace jumper, reinsert power cord and restart. Had to change a few configs in bios based on computer configuration after restart. Hope it works for you. Seen same problem on optiplex 750 and 755 in last week.

the hardrives are not for that machine.they must be formatted

dell 755 drivers page

your looking at the wrong firmware

optiplex 755 current bios version: A10 date 6/20/2008

for the CD Drive need to open device manager and check

what brand and model you have for dell used 2 DVD drives
sony and teac

use the correct firmware on drive, if not the drive will be killed.
if unsure open case and look for name/model on sticker of drive

on the back of each device their should be a jumper on the right side next to where the ribbon cable plugs in. Remove the jumpers and replace them into the slot thats for "Cable Select" Their should be a picture on the top of the device if they aren't labeled. You can also put the jumper for 1 as "Master" and the other as "Slave"