Our campaign did not send any emails today, what would be the reason for this?

99% of the time this happens when you start a campaign after it’s Delivery Window is already over. Check the Delivery Windows of your email versions at each step to make sure they are what you expect. The default is 9am-5pm every day.

Other times it may be that there are no emails to be sent today given the days between follow ups.

The remaining 1% of the time, this is caused by mailbox sends being throttled. You should contact your email provider to unlock the mailbox.