Worst case scenario, move your speakers over to the amp, unless it's a mono-block just for subs, then you will have to buy a different one... You will notice a HUGE change in both sound quality and volume if you get a decent amp onto the speakers
As for the actual problem, you say everything works, it plays through CDs and you can tell it is reading the data on the visual EQ... if you then hook up a speaker to it, and check that your levels and faders are alright (which unless you changed them they should be) and there is no sound, then the amp in the headunit is farked one way or another... could be a lost connection, dry joints, fried board... anything really... if it is under warranty, you will probably have to take it back... if it isn't and you want to keep it, you can still pay to have it repaired, but if it is a cheap head unit, it may not be worth it
K home time nao
Double check everything... it's often something silly, but I just skipped all the obvious stuff