Gray screen on iMac running Yosemite

Any suggestions on getting thru gray screen at start-up? I have performed the following: Safe Boot, zapped PRAM, and reseated & changed out memory boards. I do not have a OS start up disc. I get logo, progress bar & spinning gear. This is a late 2009, 27" iMac i7 2.88 ghz running Yosemite.

There is no install disk with Yosemite unless you made an install USB flash drive. Try starting your iMac in Recovery mode. Restart and hold down your command and r keys at the same time. That should get you into recovery where you can use Utilities, Disk Utility, to verify the hard drive.