Stuck at Level 55

Are you a subscriber or preferred? It sounds like you're a preferred player that only has Rise of the Hutt Cartel. You will need to subscribe to obtain access to Shadow of Revan to get you to 60 and Knights of the Fallen Empire to get you to 65. Both expansions are free with a one month subscription.

If you are already a subscriber you will have to submit an in-game ticket or call them to resolve the issue as to why you have not yet been granted the expansions. 

If you account isn't flagged for certain xpacs, you can't get past certain levels.  if you are stuck at SoR, you can't get past 55.  Sub should grant SoR, but it dosn't seem to be granting atm, so you might have that issue.