Dear friend,
Your details are inadequate. How long you served in this co., in what position ? Are you under probation ? How did you know that you have to give 15 days notice?
However, whatsoever may the circumstances, it won't sound well stopping without giving your formal resignation. Unless it is submitted you don't sever your ties with this co. To have a good moral feeling, peace send your resignation under proper ack. Say clearly you have nothing to hand over on your resigning. Haven't asked for the Apptt.letter or PF while in serivce, why? You did receive your salary, Legally you were in service then how you have to get out ? Did you resign or was dismissed/terminated ? How you are going to fill your CV ?
kumar.s.