Often, the error will provide an IP Address or Server name which it is failing to connect to.
With this information, the first port of call should be to reach out to your IT team to establish if the server with the XLCubed database is having any issues.
The server may well be offline and needs to be turned on to reestablish the connection.
The security setting on the server maybe also have changed and the connection details in Excel need to be updated to have access to read the SQL Server data (your IT should be able to provide details with this level of permissions)
It might be that the server IP address or name has changed and the connection will need to target this new location.
For both of those last two you will need to change the details within Excel:
Navigate to the XLCubed tab
Then select Connections
This will open all recent connections, select New Connections
Populate the new connection with the required details
Select 'Connect'