Why can my rear derailer no longer shift between all 8 gears?
It sounds like you may have bent your deraileur hanger.

Looking forward from the rear of the bike, the hanger should be perpendicular to the ground when the bicycle is standing straight up.  

Some bikes have replaceable hangers.... For others there are tools for aligning bent derailleur hangers....  Absent the tool and depending on your comfort/mechanical skill level it is possible to align the derailleur hanger with a large adjustable wrench by removing the derailleur and rear wheel and applying gentle force to return it to its perpendicular position.  

If it is bent to severely, I highly recommend taking it to a shop.