Dell Optiplex 755 SFF issue

This is a memory error.

Unplug the system, hold the power button for 30 sec and remove and reinstall the memory module(s).

If that doesn't clear the error, try one memory module at a time in each socket until you find the bad module -- or, if more than one module fails to work in a single socket - the bad socket -- and replace the corresponding faulty part -- the memory module, or in the case of socket failure, replace the mainboard.

Are you the school's IT person?

3,4  = Memory modules are detected, but a memory failure has occurred.

downloads.dell.com/.../optiplex-755_user's guide_en-us.pdf

What color is power button and is it steady or blinking? And are there any beeps from the PC?

Power off, unplug and press/hold power button for ~15 sec. Open case and remove motherboard battery. Press/hold power button for ~30 sec. Carefully reseat RAM modules in their slots and use some canned air to blow out all the dust. Reinstall the battery (may be time for a fresh one) and close up. See if it boots now...

EDIT: ejn types faster than I do. :emotion-2: