Maybe it was something funny that you saw happening; or perhaps a friend of yours said something absolutely hilarious. It could've happened in your own language, or in a language you're learning, or it could even have been an extremely gut-busting situation which needed no words at all,
I recently asked an acquaintance whether he'd seen the movie "Cast Away". And he replied "Oh, yeah, that film where Tom Hanks spends two hours talking to a coconut!", whereupon I involuntarily spat out my coffee right into his face.
Go on, share with us: what made you (really, truly, genuinely, ACTUALLY) laugh out loud?