When a company plans to do work to fix a system, they are going to do "scheduled maintenance".

A more formal verb that you can use with this phrase is "perform":

Sorry for the inconvenience. We are performing scheduled maintenance.

This phrase is common for talking about websites and computer networks.

Maintenance that is not planned ahead of time is "unplanned":

We're performing unplanned maintenance.

Or you might write something like this for an unexpected problem:

Our service is unavailable. We are working to resolve the issue as soon as possible.