So the MacBook Pro is stuck on the silver apple screen?
Can you boot into recovery mode ok? To do that you hold down the Apple key and R at the same time after restarting until you see the spinning gear. Then from there go into Disk Utility and repair disk and disk permissions for "Macintosh HD" (select it in the list on the left). If the computer is stuck at the silver screen, hold down the power button until the computer goes off and then boot holding down Apple+R. The recovery mode may not work if your system is too old, in that case you will need the appropriate OS X install disc and I'd need to know the model number to find the correct OS version.
If that doesn't work, you might need to do a format and reinstall. To do that, in Disk Utility select Macintosh HD and go to the Erase tab and erase the drive. Then close Disk Utility and reinstall OS X by following the onscreen prompts. Now this will remove ALL data - so make sure it's what you want to do and know the implications of doing an erase and install of the OS but it's guaranteed to fix your problem if your computer's hardware is intact.
I'd be interested in knowing the problem with the iMac.