First of all if you are not technically inclined then there is nothing you can really do as with the chain of events you describe it strongly suggests that the load imposed by the extra speakers has damaged the output stage. Its really a case of you checking what the impedance rating is on each of the sockets and working out if you have connected devices (speakers) under this, as if you have that will cause the output stage to draw more current which in turn will cause it to overheat and possibly fail. It show be appreciated that running with a slightly higher load (less resistance) than is specified will not necessarily blow the fuse and so be much more damaging because of this, as the fuse is intended to protect against near complete short circuits which will cause a fuse to blow that quickly that the components dont get time to overheat. Could of course be another reason altogether, but the aforementioned is by far the most commonly experienced one.