|Technical Specifications: Dell? OptiPlex? GX150 System User's Guide|
Several power supply sizes.
If you search the Dell site using the Service Tag code on the back of the case, you will probably find more specific info about your model.
Solving Problems: Dell? OptiPlex? GX150 System User's Guide
Search for: Back Panel Diagnostic LED Codes
See response 2 in this - try cleaning the contacts on the ram modules, and making sure the modules are properly seated:
If this problem occurred after you changed which ram you have installed, or if it had no ram in it and you have just installed ram, not all ram is compatible with your mboard.
e.g. You must use at least PC100 SDram if you have a Celeron cpu, or PC133 SDram if you have either a Celeron or Pentium cpu.
If you still have the ram that worked fine in the mboard before, try installing just that ram.
It is easy to test for whether incompatible ram has caused your mboard to fail to boot, or it's caused by another problem.
Make sure you have a speaker or speakers or the equivalent connected to the mboard so you can hear mboard beeps (see your mboard manual if you need to).
Remove the AC power to the case/power supply.
Remove all the ram.
Restore AC power.
Try to boot.
If nothing else is wrong, you will get no video but you will hear a pattern of beeps that indicate no ram is installed, or a ram problem.
E.g. for an Award bios or a bios based on one, that's often a beep of about a half second, silence for a half second, a beep of about a half second, silence for a half second, continuously.
Failing power supplies are common and can cause your symptoms.
Check your PS.
They often partially work, fans and hard drives may spin, leds may come on, yet you may get no video and the mboard will not boot all the way.
See response 4 in this: