Reinstall and then get a new activation. This is a common problem and Autodesk will be happy to issue you another activation code. However, you may have to get it manually.
You can use the online method to activate your license here:
Or you can can call Autodesk Product Registration and Authorization:
1-800-551-1490 > Option 1 (English) > Option 1 > (Stand-alone)
Asia / Pacific: 65.6735.5688
Europe / Middle East / Asia: 41.32.723.9595
You will need to provide your serial number and the request code.
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