If your paypal recurring payments are cancelled, either by yourself or lack of funds in your paypal account your feeds will be suspended on your renewal day.

To upgrade back to paid click the link in your account which says 'click here to update to a paid account'

Your Username and Subscription Id will remain the same.
No refunds are given on monthly subscriptions. 
Suspended accounts may be deleted completely, in which case you would need to re-register.