I would first reset the PRAM and SMC:
Resetting the System Management Controller (SMC)
Note: Portable computers that have a battery you should not remove on your own include MacBook Pro (Early 2009) and later, all models of MacBook Air, and MacBook (Late 2009).
- Shut down the computer.
- Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.
- On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
- Release all the keys and the power button at the same time.
- Press the power button to turn on the computer.
Note: The LED on the MagSafe power adapter may change states or temporarily turn off when you reset the SMC.
- Shut down your Mac.
- Locate the following keys on the keyboard: Command (⌘), Option, P, and R. You will need to hold these keys down simultaneously in step 4.
- Turn on the computer.
- Press and hold the Command-Option-P-R keys before the gray screen appears.
- Hold the keys down until the computer restarts and you hear the startup sound for the second time.
- Release the keys.
If those don't solve it, then I would do a Safe Boot:
To perform a Safe Boot, press and hold the Shift key immediately after your Mac starts up. Continue holding down the Shift key until a gray Apple logo appears on the screen. Here's further info on Safe Boot:
Hi. I have tried all these and still cannot boot. I still don't progress beyond the grey screen even if left for an hour or so. I tried the safe boot with cmd V as well. The last line of this mentioned not finding a boot cache file. I am currently trying to reinstall the os again so cant check the full directory of the cache file. Peter