My iMac won't start up.

When you say won't start up, what do you mean? There's a lot of permutations to what that sentence means.

Where does it freeze? Have you installed all available software updates?

System Requirements for OS X Lion:

• x86-64 cpu (Macs with an Intel Core 2 Duo, Intel Core i3, Intel Core i5, Intel Core i7, or Xeon processor)
• At least 2GB of RAM
• Mac OS X 10.6.6 or later (Mac OS X 10.6.8 is recommended)
• At least 7 GB of free hard drive space

Try these steps:

http://support.apple.com/kb/PH4405 PRAM

http://support.apple.com/kb/HT3964 SMC

http://support.apple.com/kb/PH4373 Safe Mode (It may take a while, but wait for the progress bar to fill up and see if it still freezes in safe mode)

If Safe Mode is fine but normal boot is slow or freezes, then try these two links:

http://support.apple.com/kb/PH4602 - http://support.apple.com/kb/PH4333 Login Items (Users)

Also, unplug any peripheral devices that aren't needed, just leave power, keyboard and mouse, see if that helps booting.

Hold down ALT (option key) when starting up, select the Recovery Partition, then select Disk Utility and verify/repair the disk, as well as verify/repair the disk permissions.

Could also try running the Apple Hardware Test, follow this article: http://support.apple.com/kb/PH4510

Post back with more information if the iMac is still playing up.

Best of luck!

MiiCK.