is there power going to the PC? are there lights on the pc? what do you mean exactly by nothing happens? does it do anything at all does it beep at all? Have you tried unplugging it from the power and replugging it? Are there fans running in the PC? If nothing at all is working then it could be a bad power supply. Does the monitor power up?

Yes I have power, lights, fan is running, can access internet. But what my problem is is that in the lower left corner of my screen, I could hit the circle with the windows icon to go to accessories, games, the sleep/hibernate/restart options, etc. But today when I try to access it, it's not coming up. If I hover my mouse over it, the word start appears. I just don't know why it's not working like it used to. Thanks.

Can you use the Windows key on the keyboard to bring up the start menu? If so, bring up the start menu that way and do a shut down. If you can't access it that way, hit ctl-alt-delete. In windows 7, you'll have the option of a red button in the lower right corner to shut down. In XP, you can start the task manager (you may need to click start task manager or it may launch when you hit ctl-alt-del) and shut down from there.

Once it powers down, start it again and see if that takes care of it.

probably the first thing I would try is a simple system restore to the day before this started it should reset your settings back. Very simple to do. I do not know if you are using xp vista or 7 the directions are different. Here is for xp
System Restore

if you use one of the others just google system restore vista or 7 which ever.

Thanks all...I don't know what was going on, but it works now!

If something like that happens again you might try shutting the computer down entirely then restarting. Often that will clear things up.