Both are fine (operation or operations) Some people when they are talking will envision in their minds all the various things happening at work as "one thing"/"one operation" so they would use the uncountable form (operation) and say '... resume it's normal operation'. Other people may think more of all the various different things happening as 'different operations', so they would use the plural countable form (operations) and say '...resume it's normal operations'.

I would use operations with the s. Maybe someone can give you a better grammatical explanation. Operation' is an uncountable noun for the process; 'operations' is the plural countable noun for the various manifestations of 'operation'.