Computer not displaying

Hello,

So my computer is running but nothing is displaying on the screen.

I know there are already answers to this but i didn't quite understand and i think it's better to express my problem better.

My brother and I unplugged my pc to dust the inside and put thermal paste on. However, once the heat sink was off we couldn't get it back on. So we took it apart to see gow it goes. When we finished we put it together and plugged it in.

No the computer runs but nothing shows. The fans and lights turn on so power is getting to it. And it is not the monitor because i tried another one and it didn't work. The fans run slower than they did and the startup beep is gone.

Is there a way to fix this. Please help.

Thank you!