Is your coffee tasting a bit stale because of old coffee caught in the grinder? Or did you use it for something it wasn't intended for, such as grinding oats and now you are left with ground coffee a la oats? Cleaning your coffee grinder is pretty simple.
Take a couple of handfuls of rice and grind it through your coffee grinder. The rice will pick up stray bits of coffee grounds - or whatever else you happened to grind in it - as well oil from the beans. It will also absorb the stale coffee smell that permeates many grinders.
Throw out the ground rice, then unplug the grinder to wipe down the inside of it. Repeat with another couple handfuls of rice if neccessary. Your coffee grinder should be as good as new!