Okay so here's my problem. My computer monitor was working but the tower would turn off after 5 minutes of use and I thought it was over heating so I turned it off and let it cool down. After a couple hours I tried to turn it back on and the tower turns on and the fan starts up but the monitor is saying it has no signal. This issue had occurred last night but after some time it started working again. I tried to give it some time but the issue will not fix anymore. I checked the inside of the tower to make sure everything is in securely. What do you think this could be or what could have caused it?

Both the monitor and computer need to first be independently verified on another known good monitor and computer to localize the problem to one device or the other. Then troubleshooting can begin on the problem device.

Some common computer scenarios that can result in this symptom:

Bad video card or motherboard.
Bad power supply.
Other bad component device resulting in POST (http://en.wikipedia.org/wiki/Power-on_self-test) not completing.
Memory modules that need to be reseated to improve electrical connectivity - often happens after computer is moved or transported.

My friends monitor is showing no signal, so wats the problem and den cpu is working properly and the mouse,key board is not working .

my frnds monitor is showing no signal, so wats the problem

If you are the original poster using another user name, then all that's been verified at this point is that it is not your monitor. It can be any of the components I mentioned in my first post. Next reseat the video card and try to boot and then if no joy verify it by swapping it out with another one.