Need some help?!?!?

So a couple of months ago the box fan on my computer stopped working and my computer turned off and every time I would turn my computer on it would tell me it's overheating and turn off. So today I took the box fan out of another computer I had laying around and put it into the broken computer the computer now attempts to boot up but nothing appears on the screen and when I try to turn it off it won't turn off unless I remove the main power supply...can anyone help me or have any ideas?