Try using another email address to sign in, the cause for this is almost always that the user has multiple FormsCentral (AdobeID) accounts and is signing into a different account.
One more possibility is that the form was shared with you and the author has removed the share. Forms are not deleted from your account unless you explicitely delete them.
If you are certain the account/email address is correct and the form was not shared with you, do you have a URL to the form you can email to me to look into it? firstname.lastname@example.org
That's it. Thanks Josh, I had used another username to open this account and create the form. I misread the email reply sent by Adobe as I'd copied it into my other email inbox and then forgot I'd used a different username. Suffice to say it works now - thanks very much. I just feel a little sheepish now...
Great, glad it is resolved. Don't feel bad, the reason I mentioned the solution is that it happens often.