Once you open it you'll see the bare speaker inside and 2 copper wires going into the fabric of the speaker and other end towards a metal plate. Figure out which wire is causing the problem and go from there. Try not to yank at the wires going into the fabric. All you need is another copper wire to make a duplicate route to nearest to fabric and other end to as far out as you can and see which of the 2 wires is the problem.
There is also another possibility - and it could be the electronics part of the amp dying on you.
How much time do you want to put into this? Most of the time crappy speakers cannot be fixed.