How can I fix a blank blue screen?

There are several things you could try, as indicated by those mentioned in this Apple Support

article about how to resolve startup issues, and perform disk maintenance. Read through them:

•Resolve startup issues and perform disk maintenance with Disk Utility and fsck - Apple Support

If you have the install-restore DVD for the system running in the Mac, that may help to start up

from and see if Disk Utility's repair disk could help. Of course, if there is a problem in hardware

a replacement hard disk drive may be necessary because these parts wear out.

A slightly different set of symptoms appear other Support articles about startup issues, but some

of the ideas are the same when attempting to resolve the cause at a user-level. If issues are too

difficult, you may choose to locate an authorized Apple service provider to have a professional

diagnostic test performed to determine if other things are behind the problematic Mac.

•Mac OS X: Gray screen appears during startup - Apple Support

•Your Mac won't start up in Mac OS X (Mac OS X Panther 10.3.9 or earlier) - Apple Support

(for later 10.4, 10.5, 10.6, 10.7 etc systems, see 'Gray screen appears on startup' above)

Links in these support pages (blue) are similar:

•OS X Mountain Lion: If you see a blue screen at startup

•OS X Mavericks: If you see a blue screen at startup

If you have an Intel-based Mac, some of the issues may be different than those with PowerPC

hardware; so it helps to be sure you have correctly identified the computer & its working OS X.

Good luck & happy computing! 🙂

iMacs have been in continuous production since 1998 in lterally hundreds of variants. We can give more specific help if yo uhca tell us what version you have.