Mine have gone into mine too before.
Is the filter on, and could he get sucked into anything in there?
Can you shine a light into it somehow?
If i were you, i would shine a light into where he is, turn the rest of tanks lights off, and he should, in theory, go into the darker place (hence why he went into the filter...)
When he comes out, then block however he got in, and make some sort of a hide for him, or if theres one in there now, make it better
Using plants, and/or bogwood is best
Thats if he can get out, if not, then you will need to get him out manually, which could be abit harder...
Is there a way for him to come out at the top, because you could intise him up, with food, then get him, and keep him in a bucket/container until you have blocked the enterace points.
Generally though, if he/she can get in, he/she can get out...It may not want to though, hence the above techniques.
Good luck, and try not to panic
Mikey