Can't boot up - stuck at spinning wheel against grey background

"Right you don't have a HD problem you have an OS problem. You have a virus or corrupt/missing boot file.

Try a Single User boot to force file check. (Command-S) It should boot to a black screen with scrolling white text... that text roll until it stops.

When it does you will see At the some text about /sbin/fsck -fy when the cursor pauses blinking type:

/sbin/fsck -fy (that is a single space between fsck and the -fy)

Press Return. fsck will go through five ""phases"" and then return information about your disk's use and fragmentation. Once it finishes, it'll display this message if no issue is found:

The volume (name_of_volume) appears to be OK

If fsck found issues and has altered, repaired, or fixed anything, it will display this message:

***** FILE SYSTEM WAS MODIFIED *****

Important: If this message appears, repeat the fsck command you typed in step 2 until fsck tells you that your volume appears to be OK (first-pass repairs may uncover additional issues, so this is a normal thing that happens).

When fsck reports that your volume is OK, type reboot at the prompt and then press Return.

At reboot if you see a dark elliptical progress bar that suddenly disappears and/or hear a tone DO NOTHING for a hour. Your EFI might be repairing itself. If you halt this process you can really screw things up (from some of your description you may have already don this). If thats the case only someone with a working Mac or an Apple Genius can help you.

If this answer is acceptable please remember to return and mark it."

hi, im having problem too with my mac air. it suddenly went hanging , my cursor just stucked and i cant do anything to it even aftr pressing any keys. so i tried restarting it but then it got stucked on apple logo screen, then i tried to press the option+command+p+r keys but the screen keep on turning on and off with the chimes sound. what should i do? pls help .