I've never encountered this myself but there are several options you could probably take. One is to try to push it back in gently with the eraser tip of a pencil, then tuck the tail in (to prevent a relapse) by wrapping rubber bands over its shell. The other option is to leave it be - I have heard of these prolapses going back in by themselves, even after a couple of weeks. Personally, I would probably just leave it be provided it's being kept by itself.
-Stephen C

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