SD cards do fail sometimes (I recently had a not very old, lightly-used Samsung card fail totally), so it is possible that this is the card dying. Are there any other messages when the card unmount?
I assume that you've tried removing and replacing the card. If not, give it a shot.
Then my first advice would be to back the card up. Stick it in a card reader and back the whole thing up on a computer. That way if you have to try something like reformatting the card there will be no data lost. If the computer cannot read it then there is plenty of file recovery software that can help with that (if you use Windows then Recuva is often recommended, and is free).
Secondly, how old is the card, and have you checked that it is what it says it is (e.g. running h2testw on a PC, or
SD Insight on the phone)? There are a lot of fake cards out there, and these are often problematic in various ways (the big one generally being that they don't really have the capacity they pretend to have, which leads to data corruption once you reach the real capacity of the card). If the card doesn't check out as really being 32GB then that would be the problem, though self-unmounting isn't normal in this case.
If you have another card, you could check that that works in the phone. That would at least eliminate the phone as the cause of the problem (or identify it as the cause, but let's hope not because a card problem is cheaper to fix). Testing the card in another device, if you have one, may also be worth a shot.
Once you have the card backed up and checked out, and have tried standard tricks like removing the battery from the phone for a couple of minutes (if it has a removable battery, as a way of ensuring the RAM is cleared), then if it's still not working you could try reformatting the card then copying the data back to it. That
might help for some problems. But no guarantees: it's very hard to know from here what the actual cause of this is.
