Is the sending address a custom domain address. Free email addresses don't allow sending and may cause failed emails. You can check in your Sequence Settings for this. 

Are there Subscribers currently active on the sequence? If not, then the Sequence won't send yet!

Are the Sequence emails published? You'd be surprised how often this happens.

Is the Sequence able to send on the set sending days?

Were last minute changes made? Any last-minute edits or changes will cause the Sequence emails may to out the next day.

Are there any exclusions in the Sequence (individual email exclusions, or Sequence settings exclusions)? Exclusions are great when used minimally. If you aren't seeing your emails send, this is a great place to check. 

Were Subscribers added to the Sequence through a Visual Automation before the Sequence was published? Those users would now be 'unsubscribed' or 'completed' from that Sequence and will not receive any of the emails.

Is the Sequence part of a Visual Automation that is paused? They will not resume getting emails until it is published.