My MacBook air turns on but I can't get past the gray apple screen

SOURCE: completely white/gray screen

First try this: shut it down, disconnect the power cord, pull out the battery, then hold the power button for a full 10 seconds. Reinstall the battery, reattach the power cord, turn it back on.
Still doing it? Put the reboot disk that your Mac came with, or the disk to the version of Mac OS X that you're running.
To start from disk hold "C" as your Mac is first turning on from a complete shutdown.
Dan Crowley, iKnowApple.com 919.280.3101

SOURCE: Turning on my macbook- all i see is white screen and 'no' sign

Apple has a troubleshooting document about this here: http://support.apple.com/kb/TS1892
Normal procedure is to reset the NVRAM and PRAM then "remind" your Mac where to find the system folder. (This problem sometimes happens when the system folder is moved, renamed or deleted.)
It may be necessary to reinstall the operating system. (Fortunately, if you use the "archive and install" option, you can do this without erasing the hard drive or losing your user data.)

SOURCE: My Macbook shut down during reserve battery power

If that does not work, when you turn on the computer, hold the "Command" and "S" keys, this should boot to single user mode. When finished, it will dump you to a command prompt. From there type "/sbin/fsck -fy" (no quotes) and press enter. That wil run the BSD Filesystem Check. Once that is complete, type "reboot" and hit enter.

If there was an error with the file system that was keeping you from booting, that will solve it.

SOURCE: Macbook Pro goes from gray apple screen to black

Try booting into single user mode by pressing Command+S while booting. You should then be at a prompt. Type in:
fsck -f
and press Return. Let it run and see if it corrects any filesystem issues, then reboot when it's done.

Damien Gole

• 221 Answers
• Posted on May 29, 2009

SOURCE: at start up, the screen freezes, white with a gray apple insignia, and a timer that never stops

Hello I agree with benwabnick, I would also try to repair disk permissions and volume using the disk utility tool that is on the install DVD (hold the C key to boot from DVD, then choose english language, go to utilities, and choose disk utility).