So basically I fell off my bike and my chain fell of but as soon as I put the chain back on and thought it was perfect I noticed it stayed in gear 6 even if the numbers changed to for example gear 4 it was still on gear 6 so then I put my bike upside down so I could get to the chain easier and then when I put the chain on the gear 5 as soon as I put my hand on the pedal and started pedalling it dropped back into gear 6. What do I do to stop this?
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.