Service tax not known to be paid and now penalty is huge

My dad has a hotel in Lonvla n we had given it on rent to some guy before 6 years that too for 5 years we gave on rent . And we din know that we have to pay government service tax also separately . And now after 5-6 years government has made a huge bill on us stating that we need to pay service tax for these many years and we din know only , we were not aware of . What do we do now cos it's a huge amount and our CA or no one told us about this tax .
Nor did the govt inform us about this earlier

please help please