Hi:
This almost always means that you are looking at the wrong Google/Gmail account; one other than the account that holds your Google Voice phone number.
If you used the Google Voice "I forgot my user name" tool, there are only two possible responses:
If it says it sent an email note, then yes, your Google Voice number is active/working. It knows which Google account owns the number, and it sends an email to the correct account. There is no way for it to send a note to the wrong account. You need to find the note in some other account.
If it says that the number you entered is not a valid Google Voice number, then the number was deleted or reclaimed for lack of use. Note that "use" is defined as making outbound calls or sending outbound text messages. Simply receiving inbound calls doesn't count.
http://www.google.com/support/voice/bin/answer.py?hl=en&answer=115100&topic=29345