If new batteries will not help, do a simple test: does the MCP "wake up" by just turning the wheel? If it does, but still doesn't move the volume bar, you probably have a problem with the optical encoder connected to the wheel. Not impossible to fix (and should not be too expensive either) but will probably require a serviceman familiar with these units.
If it doesn't wake up from the wheel, there are a couple of contacts which may need cleaning. This is an easier job.
I agree the batteries lasted a long time if you have never replaced them I think the display should start to blink when the batteries get weak, but I haven't had my MCP long enough to find out for sure - I saw it once, but have since been using it for at least a month or two... it's not my main remote though.