Even though the cable/housing seems OK it's best to pull, inspect and make sure it's working at its best. There should be no corrosion on the cables where they are in or near the housing. If so replace the cable. It helps as well to spray out any contamination in the housing with WD-40 or similar. Then feed the cable back in, applying a medium oil (not grease!) to the cable as the portion that will be in each section of housing is inserted.

Once that is done check the rear derailleur itself, as you mentioned the problem occurred after a fall. Check especially the upper pulley to make sure it does not have excessive side play. You may have to find a matching model to your derailleur for comparison. If the pulley moves too much you might end up tweaking the adjustment inward slightly (to make the shift to large cogs) which then doubly compromises the shift in the other direction. Also check the pulley and main derailleur pivots to make sure there is no play at all. Finally, although more rare it is possible for the derailleur body or pulley assembly to be bent in a fall. Check the derailleur pulley assembly from the rear as you shift through the cogs and on different chainwheels. It should neither lean in or out no should it orient to the right or left of parallel. Actually, the definitive way to determine if the derailleur is the problem is to mount a different one that is in good condition.

It's unlikely to be the sole problem, but as with the cable it's best to be going for the best operation possible, so also check the chain for wear. As chains develop longitudinal wear they also tend to flex more laterally, which can degrade shifting, and would have a similar effect as a bad top pulley.