Campaigns are made inactive if no mails have been sent from them for a period of six months. To reactivate the campaign, go to [setup] → [campaigns] and click the green checkbox. You should also send an email to a group to make sure the campaign is not automatically deactivated again the next day.
After 2 years of being inactive, a campaign is deleted from the platform. Before this happens, you will see a warning message on the home screen. After this happens, the only way to reinstate a campaign is for Tripolis to restore it from a backup, but after a certain period this option will also no longer be available.
To prevent any of this from happening, you can change the setting "Deletable" to "No" in the campaign details in Setup.