Some more details will probably be required. The OS X version will help, as a start.
If you have the diagnostics DVD disk, boot that and see if there are any errors logged.
If this is OS X that's installed on a MacBook Air that you're using, then some of the usual triggers and approaches for resolving the still waiting for root device errors are discussed here. Have a look at that, and see if something in that sequence looks familiar, whether the PRAM reset or the safe boot helps or works here.
If there's any anti-malware, I/O accellerator, antivirus tools or similar installed, get rid of it. At least for testing.
There's a mention of "other OS" in your footer. If not OS X, what is the operating system in use here? If this is some other hardware or software involved, then the triggers for this error can vary. Widely.