Power has been restored to 350 City Hall Square in Windsor but 400 City Hall Square remains closed.
Both buildings were closed all day Tuesday after power was cut.
City officials initially thought the cause was a blown transformer but investigators say a cable failed in an underground conduit.
Services impacted by the closure of the 400 building include the Provincial Offences Courts, Service Ontario and Service Canada.
The city says municipal employees who normally work in the 400 building are asked to work from home, where possible.