Do you have a “Volume Limit” set for that Room?

Do you have any voice control or other controls enabled for that Room?

Yes calling the back yard a Room is silly but that is the name Sonos is now using instead of Zone.

No I don't 

Is this a new development? Has the “Room” worked OK in the past? What model speaker(s) are you using? Are there any Volume controls or switches?

Yes everything was working good havnt used it in a few weeks and wanted to use and now volume keeps going down to 15%

AMP may be reacting to overcurrent. AMP will protect itself if the circuit load is excessive. Outside wires and speakers are subject to damage by weather, critters, and construction. If you have access to an ohmmeter, check the connection resistance and also check the resistance to building ground.