Ok. Here are the instructions to follow to try and solve this problem:
1. First Exit Lion Installation.
2. Then go to applications folder.
3. Find the file name "Install Mac OS X Lion" and Delete it.Then restart your mac.
4. Go to the app store.
5. Go to purchases and click redownload Os X Lion.
If It Shows that it is already installed then press and hold the option key when you click on the purchase tab. If It still doesn't happen then use spotlight to search and install the mac OS Lion.
Hope this helps