When you downgrade your Pro plan to one of our other plans, your Overdub voice license is revoked. After re-subscribing to a Pro plan you will need to re-share the Overdub voice with your account.
You can do this by opening a new Drive View (File > Open Drive View), select the Overdub tab on the left sidebar, click on your voice, then select Share voice. There you will have the option of re-assigning the license to yourself.
Users with multiple drive plans on their account will want to make sure they select the correct billing drive from the list and then their User will become accessible. If a drive is marked with a symbol, it does not have Pro-level access and would need to be upgraded.