lion start up stuck on apple logo and the spining circle
i have intalled lion sucsessfully the day it was rellesed and it worked well since then,
yesterday i shut it down and it doesn't boot it's just stuck on apple logo and the spinning circle in the middle
please help
i have an important project should of been done before this weekend
my device is imac 21.5" with i3 proccesser
8 gigabyte ram memory and 500gb hard drive
iMac, Mac OS X (10.7.1)