Could be either your radio has decided to blow a channel (pretty unlikely) or one of your speakers has gone.
If it's an aftermarket system it could be a dodgy install and a connector has fallen off, or an amp that has buggered up, or a dodgy RCA cable, it could be anything.
As a start, you could take your door apart and test the speaker with a multimeter. Set it to ohms and the speaker should read (between terminals) about 4 ohms i think. I would say on the back. If it's 0 or really high, get yourself a set of speakers from JB Hifi or autobarn pretty cheap and install those. Alternatively, they might install them for you. VZ doors are 6 inch speakers and its pretty much one size fits all.
