bottom half of screen not working

iMac model? Mac OSX version? iMacs have been in continuous production since 1998 in four broad families, each with different service and troubleshooting procedures.

Do "About this Mac" from your Apple menu will show your OSX version ("iOS" is not a computer operating system; just for Apple mobile devices) and processor type. Please tell us what you find for those two items. The latter will at least narrow down your iMac to one family.

Shutdown your machine. Hold down the shift key. Poweron. Wait awhile Wait awhile while you harddrive is being checked.

http://support.apple.com/kb/ht1455