Two things come to mind. Check the shifter linkage on the left side of the bike - if it's loose, it may have unthreaded itself to the point where it's interfering with something and physically stuck. If that's the case you can probably fix it yourself in about 15 seconds with an 8 or 10mm wrench.
The other thing is a bent fork in the transmission ... it's unlikely, but expensive to fix.
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