It depends on what are you using.
Assuming that you will use gcm without a third party server:
gcm token do expire in some cases: http://stackoverflow.com/questions/31061354/when-do-gcm-tokens-expire-and-what-is-the-instanceid
No panic, just refresh the token. All you need to do is ask for a new one. Your app can do it periodically and send it to your server.