I just pick up where I left off.
A common interruption would be a phone call in the middle of praying the rosary.
“Hail Mary, full of grace…” Ring. I answer the phone. I keep my finger on prayer bead.
Wrong number. “Hail Mary, full of grace…” as if nothing happened.
I have been interrupted while praying walking down the street. “Hail Mary, full of grace…”
“Could you give me directions to…” Gave directions. “Hail Mary, full of grace…”
No, you do not need to start over with the Sign of the Cross. Just continue as if nothing happened. Simply return to where you were. The importance of prayer is that your heart and mind are on God. You have merely excused yourself for a few seconds to answer the phone, the door, or to give somebody directions.