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.