Currently Being ModeratedDec 3, 2012 10:29 AM (in response to Milage12345)
Welcome to the Apple Support Communities
Press Option key and check if you can start in Macintosh HD. If not, insert the DVD that came with your iMac, press C key while your Mac is starting, go to Utilities > Disk Utility and repair the disk. If it doesn't help, take the computer to an Apple Store
Currently Being ModeratedDec 4, 2012 7:30 AM (in response to Milage12345)
Before hauling a heavy computer to an Apple store, make sure you know how old it is, or the exact model. Apple only works on computers made in roughly the last five years, and the iMacs in this forum--G3, G4, and G5 models-- were all made prior to 2006. The description of the vertical lines sounds like a G5.
This article explains what Apple does and does not fix:
Here are two articles to help you ID your model: