My pc wont start

aded some memory and the monitor wont come on or the keyboard and mouse ...i tryed removing the memory and tested that oput on another old coomputer ,,thats fine ,,my copnnections are all good ,,what next ,,,?

If it does nothing, no fans, no sound, check the power input and the power supply. You can put the probes of a voltmeter between the red and black pins of one of the spare hard drive power plugs to check the power supply.

If it makes beeping sounds like two long and one short beep. Look up the codes for that make and model and it will tell you what the problem is.

If you hear the fans but no beeps in Windows, try to boot it holding down the F-8 key, F-2 for a Mac.

If that doesn't work, borrow another monitor and try it.

Comment back here and let me know what you find out if you need more help.