You need to have paperwork showing the bike as collateral for the loan, (like banks do), you need to show that the loan is in default, and last but not least, you need to know where the bike is. You...
be late with a payment and you have violated your loan agreement: at this point the lender can repo the vehicle.They usually wait about 30-45 days to give you a chance to catch up;remember they don't...
If you want the trailer back (repossession) you definitely need to see a lawyer in your area. Any mom-and-pop "general practitioner" lawyer can do this--no need for a huge law firm.
If you don't...
Call them up and straighten it out. They will catch on anyway and it is best to be able to give them the right one. Otherwise if an "act of God" happens, you are covered.