My bike turned into a fixed gear on its own?

The last time I went for a ride was a week ago, and after quickly cleaning my drivetrain after the ride, I haven't even touched the bike since. But today when I tried testing the bike, it had for some reason converted itself into a fixed gear. Can't backpedal, and the pedals turn along with the backwheel. My ride last week did result in my drivetrain getting blasted with slushy snow and wet sand, could that be the cause?