MySQL Replication - Resync master after slave taking over
If the Master was already acting as a Slave, then you really had a Master-Master setup, and the replication from the old Slave (current Master) will flow to the old Master that had been down.

Otherwise, you are in deep weeds.  You may as well promote the currently-acting-as-Master machine to be the Master.  And then rebuild the other machine as a Slave.  Or rather a Master in Master-Master setup so you don't hit this problem again.