What's the correct tense?
It depends on the context of either the sentence before or after the sentence you've written though by just the sentence above "was restored" sounds much more plausible as the word "previously" indicates something which has already happened while instead using "is restored" indicates that which is happening now, e.g. the power is now restored is "now" restored
e.g. the power was restored It was restored which means it has been restored sometime "before" now