Threre are many reasons when a subscriptions becomes inactive. I too got into similar situation. Microsoft says

"If a subscription is not synchronized within the maximum distribution retention period (default of 72 hours) and there are changes in the distribution database that have not been delivered to the Subscriber, the subscription will be marked deactivated by the Distribution clean up job that runs on the Distributor. The subscription must be reinitialized."

This means that if you have some changes in the distributor that has to be propagated to the Subscriber and if that retention period exceeds the maximum retention period 72 hours by default, then the job Distribution clean up will make your subscriptions inactive. You can configure this retention period through right click Replication -> Configure Distributor -> Retention Period