Optplex 990 wont turn on

If you mean the PSU (not a battery) button does nothing (green LED does not light up and no fans) then that would indicate a bad PSU assuming no internal components are at fault.. Make sure the outlet or power bar that the system is plugged into is working properly first. The video on this page describes the BIST (Built In Power Test).

Also which form factor 990 do you have?

you can use a spare working 24/4 pin psu to test 990 motherboard.  also do not forget to replace cmos battery w a brand new one.

nothing happens plugged or unplugged and idk what form factors are

form factor JOCean referred to means MT. DT, SFF (the size of desktop chassis).  do you have a spare working 24/4 pin power supply unit to test the 990 and rule out bad psu?

The power button doesn't work basically no power gets to it I think

pc just doesn't work. 

I don't have any pin power supply units. DT I think

First try replace cmos battery with a brand new CR2032 one then clear cmos settings by reset jumper (RTCRST) on motherboard.  do these simple tests before thinking of replacing psu.