Well, if it really is the speaker only, you can buy a replacement anywhere on the net (i.e.: Eminence Speakers - Eminence 12" speakers - Eminence Alpha, Beta, Gamma, Delta, Kappa, Delta Pro, Kappa Pro and Omega 12" speakers. Eminence 12" speakers here.) or buy/order in a local music store.
Does the speaker make any sound? You could take 9v block battery and hook it to the speakers taps. If it doesn't move the speaker's history.
If it's NOT the speaker you better take it to a qualified tech.