Something could have came loose or broke... if you can, check all the solder points in the area and make sure they're all connected. If you can't see anything wrong with it or if you see something wrong but are afraid to solder the tiny points, see if there's any local techs that handle this kind of deal.
I've heard multi-effects / modellers are expensive as hell to repair, so it may not even be worth it... worth gettin' checked out though, I could be totally wrong, I'm just going by what I've read.
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