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.
Inactive campaigns are shown in red. Click the green checkbox after the campaign name. The campaign is now active again. Then, send a definitive email to a group, not a test, 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 Spotler 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 set the campaign to not deletable. Go to Setup → Campaigns. Click the campaign and then click Edit. Change the setting Deletable to No. Click Ok to save.