My amp works fine but when i raise the volume with the speaker connected it turns off?

I have a 400watt DHD power cruiser amp connected to a 10 in Kicker competition sub. The system worked perfectly for over 9 months then suddenly it stopped. Now with the speaker disconnected the amp stays on, but when i connect the speaker it only works at low volume ( i mean low like 9/35 on the volume scale) when the amp turns off. I replaced both fuses because the one on the amp had burned out. But the system doesn't work anymore. I know very very little about stereos and electronics. Any suggestions on what my issue may be?