Once an install starts happening, it is very hard to revert it, unless you have some sort of backup.
Since 10.7 is no longer directly available from the Mac App Store, you might find it trying to restore 10.8 if your Mac supports it. If your Mac is able to boot, clone it, otherwise use the restore directions on my tip: https://discussions.apple.com/docs/DOC-1992
Once you have some semblance of a backed up system, contact the Mac App Store to ensure when you go to restore the system with command-R, that you get 10.7 if you didn't save it on a Flash drive or DVD in the first place, or if you didn't clone your 10.7 system before upgrading:
In the future always backup before upgrading.
Note:10.2 to 10.5.8 and earlier users can revert back with their 10.x installer that installed 10.x on their system. You posted to the 10.5 forum, so it only is fair to mention it. They can use various archive and install options outlined on:
An archive and install of an earlier 10.x is typically not feasable because of the differences between the systems. I.e. 10.2.x can archive and install over 10.2.8, but not 10.3 or later, so long as the 10.2.8 combo update is applied at the end.