There are several things that are likely to cause this problem. The first is the external volume slider. This is the one that you physically move with your finger. There is a tab on the inside of the slider that aligns with a tab on the internal slider. If either of those tabs are broken then they need to be replaced. If the outer one is broken then you just have to replace the outer volume slider. This would be a pretty easy job. If the inner one is broken then it would be a very hard thing to replace. It is soldered to the motherboard in a way that makes is very hard to desolder.
If I were you I would at least remove the back cover to take a look at the volume sliders and see if they are broken. It's possible that they have just fallen out of sync with each other after being dropped. If you do decide to take it apart be very careful when you remove the back case as there will be a ribbon cable that is still attached.