How do I re-use my account information on Kindle app?
To get back your @kindle email address (assuming someone else hasn't acquired it in the meanwhile), browse to Amazon.com and navigate to Your Account > Manage Your Kindle > Manage Your Devices > Personal Documents Settings (this last link is rather less than obvious, below the panel showing a photo of a KF).  There is a link there to edit/change the KF's email address.  You can do this from any computer; it doesn't have be done on your KF.