Everything on our main floor house went out. Lights, water, fridge, etc. But everything downstairs still works. We have buss fuses on the main level and a cercuit breaker box downstairs. What could be the problem?

A short circuit or overload could have opened (blown) the fuses, or they could have opened due to age or fatigue.

There are so many other reasons also, such a loose connections, etc.

If replacing the fuse does not solve the problem, you may need to call in an electrician to troubleshoot, locate, and reapir the problem.