Can't select anything-- won't light up applications or click. Help, please

Hi--

My computer isn't letting me select anything. It was working fine, I was uploading things for my Amazon bookstore, and talking on Ichat and then bam.. it wouldn't let me select anything at all, but my friend could still hear me on Ichat. I held the button down to force shutdown, and turned it back on, and my mouse was moving perfectly, but it wouldn't let me click on any applications or even empty trash. Nothing would pull up. Please help. Thank you in advance!
Myra