sounds like the latch spring is broken or the latch mechanism is frozen.
If you "cared" much, you could pull out your seat and take it all apart to get at the latch mechanism and try to repair it but it's pretty damn hard to fix it once it freezes. It's put together with a huge rivet type thing but you can drill that out and put it back together with a metal sleeve and a bolt with lock washer.
you can get springs from ace hardware.