Wires came unhooked. Speaker blew. Channel on the amp stopped working. Could be a million things. Pull it and make sure the wires are still connected to the speaker. if they aren't, there is your problem. If they are, grab a DMM, set it to AC V and put it on the speaker wires while the stereo is playing. It should read some voltage. If it doesn't, then you need to check and see if either the wires came loose at some point down the line or if the amp's channel stopped working (yes, even if it's just the amp internal in the HU). Etc Etc Etc