Projects usually become corrupted if a plugin (3rd party of course) crashes while FL Studio is saving.
If you have not loaded and saved another project since the corruption, look in -
\Program Files\Image-Line\FL Studio\Trash bin\Backup.flp or Backup.zip depending on what you saved last. This file is auto-created when you save a project. It will be the last project successfully saved.
If that fails...
There is a recovery function in the tech-support diagnostic program.
Download the recovery program by clicking on this link.
You should systematically remove each plugin in the .flp until you find the one that was causing the problem. Once the offending plugin has been deleted the .flp should load.
The FL Studio support team.