How can I fix my computer tower?

Last week while I was turning my computer on, my cat jumped on the tower and checking back on my computer, it froze my computer.

Later that day, I got it to work(with slightly messed up graphics) but only for it to freeze and shut off.

I'm pretty sure my cat didn't do anything to it.

Going through the basics:

My computer was bought sometime April/May of 2010.

It worked perfectly fine since, until last week.

I regularly scan my computer for viruses, nothing like that.

But, replugging all the cords that only work the tower, and taking the ones that don't. Does not work.(Did one time)

The LED light lights up. But checking my monitor, the LED light there just flickers.
When my tower turns on, there's usually two noises when put on. I currently only hear one, which is most likely the fan.

I'm pretty sure it's something in the tower itself. But I'm unsure on what it is. Please do help, I'd like to use it again.