is trial balance chosen in your report type and current fiscal year 

More than likely a change was made to the Chart of Accounts so the account or accounts need to be modified as being a part of a sub-total or group account or possibly a Header or Total are missing or are in the wrong place.

Yes

I don't believe any Chart of Account changes have occurred - nothing looks out of place when I review the Chart.  Just that one day the TB worked, and the next day it didn't!