That's most likely because you have set some limits, check your customer groups sending limits, and also the sending limits for all customers from backend > settings > customers.
AIf the above does not apply, more than sure you have a notification with the reason why the campaign has been blocked, again, it can be because of a higher bounce/complaint rate, a rate which you defined in the settings, so check that as well.
And lastly, campaigns can be blocked because you defined a list of words for subject/content and that campaign matched it, so check that as well.