If you are sure it is your bike and you can prove it, contact the police.
If you are sure it is your bike, but cannot prove it - sorry, you're out of luck.
If you think it is you bike and can prove it if given the opportunity to further examine the bike, contact the police.
If you think it is your bike, but cannot prove it - sorry, you're out of luck and would feel really bad if your were found to be wrong.