What would have happened if the US lost the Revolutionary War?

In the short-term, America would've remained a British colony. In the long-term, however, it's almost certain that America would've broken free eventually. The revolutionary fervor that had inspired the colonists in their fight against the British government would not have diminished over time; if anything, it would only have intensified.
That would have been hard enough for the British to deal with. But once you put their conflict with Revolutionary France into the mix, you have some idea of just how difficult it would've been for Great Britain to maintain control over her American colonies. Dealing with the ever-present threat of Revolutionary France took up virtually the whole of the British government's energy; they seriously believed that the French would invade at any moment and topple the British monarchy just as they'd done with the French.
That being the case, the British would almost certainly not have been able to devote anything like the level of resources necessary to subdue the American colonists by armed force. For this reason more than any other, the Americans almost certainly would've defeated the British eventually.