From the Model Mayhem FAQ:
How do I reactivate an account I closed?
This should be pretty simple! Just login to the account you want to reactivate, make sure your account has the required images and About Me entered, and then click the Reactivate button at the top of your My Stuff page.
If you need help reactivating an old account you can no longer login to, you will need to contact the mod team for assistance.