You cant lose it man ! Check your mail . If its not there, then mail them other details that could identify you. Good luck. 

FYI- This is not a big deal, so relax. BITS wont reject you just becuase you just because you lost your application number !! Chill. 

Mail them your name, The tentative time when u did fill up the application, Give every personal Detail possible and request them to reply you as soon as possible.

BEST OF LUCK!!!