I had that problem, but in my case the Oasis was underwater for a minute the previous week, so I think that was the problem.
I put it in a dehumidifier for 48 hours (it can be a single sealed box with a dehumidifying tablet that you can buy for ~€2/$2) and now It works again
(the first few minutes it was a little slow, but later it became fine, like the first day).
It is not necessary to use a hair dryer as others says, it can even be dangerous, the back is made of aluminum, so it can hurt your hand when it is hot
(like it happened to me :-( )