I would try doing an SMC and PRAm reset, if that doesn't work then see if it woll allow you to boot into recovery, once there verify disk permissions/repair disk permissions. If the problem presists from there re-install OS Lion (assuming you were able to get to the recovery partition.
SMC reset:
Shut off machine > Pull out the power cord from the back of the mac > While the cord is removed hold down the power button for 15 seconds > plug the cord back in and proceed to boot
PRAM reset:
Hold down Command + Option + P + R > trun on the computer and continue holding those keys down until you hear the startup chime 2-3 times
Recovery partition:
In order to boot to the recovery partition hold down the option key at startup, a screen should appear that has Mac HD and Recovery, go into recovery. From there select disk utility and this will allow you to perform first aid (I.E. repair disk, verify permissions, repair permissions)
If that does not work, again boot to recovery and select "reinstall Mac OS X"