Sorry for the silly question, but have you checked the deck height knob that is just below the seat? If that is in the highest position, that position locks it into place. That thing can be persnickety and will insist on being difficult to disengage. And, you won't be able to look under to see since the mower deck will be in the way.
The other thing that came to me is that the lift arms are somehow "hung up" on something.
Be advised, Pkailing, that I readily admit to my limited knowledge but when I saw that no one had responded to you, I felt compelled. Who knows...one of these days I'll get something right!
Hope you get it figured out.
Tractor Gal