Yes, you can reactivate a campaign that has ended by updating its 'End date' however, you need to make sure to do it before your campaign is archived.

The archiving process starts when a campaign has had no impressions in the last 30 days. Read more about how you can manage your campaigns depending on their Delivery Status.

In the case when an ended campaign is archived, you can duplicate the campaign and adjust your settings according to your needs at the time.