Weird gear ratio for a couple of seconds after crash
The crash made the chain drop from the currently selected front chainring to a smaller one (i.e if you have 3 rings and you where on the middle, it jumped on the 1st - lefmost- smaller one).

The crash didn't change any gear on the shifter though, it just made the chain pop to a smaller ring. Maybe not fully, but partially.

Afterwards cranking a couple of pedal strokes made you feel that the pedals where "moving fast". After a while though, and since the front deraileur was still in the original position (maybe middle ring), the chain moved again back to the middle ring making the gearing ratio as previous.

Dropping the chain is common in MTBs without a chain guide. What happened to you is normal.