I woke up one morning, about a year ago, with my middle finger on my right hand curled in and it took a while to unlock. After a bit of googling I assumed that Dupuytrens Contracture was the problem. Having had carpal tunnel, I was aware that prevention is better than cure.
So since that morning I have taped my middle finger up to prevent it curling into my palm. If I don't go to sleep with the tape on my finger the issue returns (although over the year it has got much milder - probably because of the tape).
It can get very bad if you do nothing, which would then necessitate an operation which might, or might not, be successful.