Fender Bassman 150 - using DI and loosing output to speaker

[praisebass]

So, I use a Fender Bassman 150 combo at church - I use the DI into the sound system and have the amp turned back toward me to use as a monitor for me and the drummer. Today, the output to the speaker stopped at some point - not sure when as someone else was playing bass while I played a different instrument.

This has happened to me once before - I had just reset everything and output to the speaker came back.

What causes this? Is is unique to this amp? Any helpful tips on fixing or avoiding the situation?

Thanks