Hi, I have a denon AVR 889 about 4 years old and the subwoofer suddenly stopped working,how can i check if its the amp or the sub thats the problem?

A Denon AVR 889 is not a sub so we don't know anything about the sub in question.

Maybe the existence of the sub is no longer set in the AVR. Look at the speaker definitions first before going to wild.

Try another cable to the sub.

OR, run the sub's signal supply cable to one of the Tape Outs on the receiver and tune in something on FM. If that works a) it will be loud, and b) go back to the SUB settings on the AVR.

Crack the sub open and access the terminals on the speaker itself. Disconnect them from internal components and run one of your amp channels directly to it. If it works, the amp or signal path are dead. If it doesn't work, it's a dead driver.