Gear keeps falling back to gear 6
Sounds like your derailleur cable broke or got loose and so your derailleur is moving to its default outermost position, the smallest gear (assuming your shifter is labeled such that 6 is the highest gear). 

You might be able to find where it's loose and retension it. More likely, you'll need a new derailleur cable. About five to ten Eurodollars. 

However, in a situation where your bike toppled, you could have bent your derailleur hanger. It's worth just having the shop do the cable replacement and rear derailleur tuneup and realignment. It should be around half an hour of shop time labor. 

Note: another scenario if you have index shifters  is that the crash yanked the derailleur with such force that it broke the index shifter internals such that they do nothing and the derailleur is reverting to default. Again, the shop should be able to tell. New shifters are only fifteen to thirty Eurodollars for the inexpensive lines.