>> My AutoCad 2011 suddenly expired.
you get what message that tells you that it expired?
If you have a student-license (the only one that I know has an expiration-date) then imho the software runs only limited time, then you have the ability to download the current release and register that newly.
Oops ... or are you speaking about the end of a 30-day test period?
Then you have to buy a license (to get a serial number and so to be able to activate it). If you already have a subscription you can login into the subscription-center and there you see your serial (http://subscription.autodesk.com)
- alfred -