In accountance sense, if you have no assets other than cash (own aircraft), you are already bancrupt.
In the game you are bancrupt when you declare bancrupcy, however bank will notify you before you become a bancrupt for them.
You can declare bancrupcy once in a game and start your airline over.
Press the GAME INFO big blue button and then Airline Statistics, and in Airline value you will see how many airlines are in debt and how deeply. With my previous try I have been -3.7 mln