Contacts cannot be re-enrolled into the same sequence more than once, as the system has a safety mechanism in place to prevent this from accidentally happening.
That said, you may simply clone a sequence, and enroll any prospects into the cloned sequence. If you are on the Scale Plan, you could take advantage of the "Enroll into New Sequence" step inside your sequence builder to enroll them into another sequence if they made it through the first one!
If you are looking to re-enroll a prospect into a sequence with a different email address than what was originally in place for the contact record, you would either need to (1) Create a new contact record using a new email address and then enroll them in the existing sequence, or (2) Clone the existing sequence and enroll them with the existing contact (with the updated email address) into the cloned sequence. These steps would be needed because we use the original email address on any record as a unique identifier.