Hey all
I'm trying to upgrade an old computer that I got from a friend, I bought a new hard drive and some more RAM. I installed the hard drive first and then booted the computer and it was all fine but when I installed the new RAM and tried booting the computer the monitor displays no signal, it is not the monitor or the cables as I have tested them, I have also tried switching the hard drive back to the old one but still no luck. I want to see if I can fix this myself before I take it to a computer repair shop.

Any help will be appreciated

Take the new RAM out, revert to the original RAM, and see if the computer operates normally. The motherboard needs to be completely de-powered and drained before removing or installing RAM. Most modern boards have a LED to indicate when the board is powered.

I agree with Scleros. You said it yourself this happened after you installed the new RAM. Your best bet is to take out that new RAM and revert to the old, at least temporarily for testing purposes.

Do you have the correct RAM?

I reverted to the original RAM but when I booted the computer it started to beep, 3 consistent beeps then it would pause and repeat.

Refer to the motherboard documentation to determine what 3 beeps indicates. The documentation is available on the board manufacturer's website. Try removing and reinstalling the original memory module. They sometimes don't make good contact after being messed with. The slots may also need to be dusted out with a computer duster.

If you have an external video card. One that isn't built into the motherboard. Remove and reinstall it and make sure its fully seated. Most memory when installed you can hear a click when fully seated so make sure its all the way in the socket and closed.

I've also experienced numerous computers not even starting due to cables not making good contact to the board. The cable might look plugged in yet all it needs is a wiggle here and there.
Unplug all cables, 1-by-1, and plug it back. Also unplug the expansion cards and plug them back.

If, no luck, unplug absolutely everything except 1 stick of RAM the 2 power supply cables to the board and the power switch. Also take out any graphics card if installed and plug your vga cable onto the onboard graphics port. Switch on and see. If still no luck remove the stick RAM and replace with another, and so forth.

But if I were to guess the most likely culprit, I would choose the power supply.

Hope this helps :)