The most common cause for this is that you're using Sendgrid's free plan. Sendgrid's free plan only allows you to send a hundred emails per day. If you are on Sendgrid's free plan and you have (a) A campaign that sends more than a hundred emails per day, or (b) multiple campaigns that are sending more than a hundred emails per day combined, then your campaigns might pause once you've exceeded the 100 emails per day limit. You'll also see an error in your campaigns list that indicates that. But normally, it will resume on the next day.