Booting in Safe Mode deletes system caches thay may help.
A Safe Mode boot definitely takes longer than a normal boot so be patient.
After you boot in Safe Mode you'll see the Desktop.
Click the Apple menu icon top left in your screen. From the drop down menu click Restart.
And check to see how much free space there is on the startup disk.
Right or control click the MacintoshHD icon. Click Get Info. In the Get Info window you will see Capacity and Available. Make sure there's a minimum of 15% free disk space.
If booting in Safe Mode didn't help and there's enough free disk space, use Lion Recovery to repair the startup disk if necessary, or reinstall the Mac OS X.
I'm trying to use Lion Recovery, but after accessing the Recovery HD (after starting with Option key pressed down), it asked me my apple ID. As I did not purchase Lion in the App store, the process stops. To make clear, I did not purchase Lion in the App store because my imac was bought in July 2011 with Lion pre-installed.
How do I reinstall Lion in this case?