If one stitch popped open, what should I do?

The best answer I can give you is to contact your plastic surgeon. If you had surgery out of state or abroad, he or she should still be available. Without seeing the wound it is impossible to evaluate, but the best way to treat a small opening is keeping it clean to avoid infection. A clean wound will usually heal itself. You can clean it with alcohol and use antibiotic ointment on it every day. Most importantly contact your surgeon. I would certainly like to know if you where one pf my patients.
Best of luck on your recovery, 
Dr. Miguel Mota

It’s always advisable to inform your surgeon if a stitch has opened up. If they’re too far to meet, you can see a medical professional closer to you who may be able to help although make sure to let your surgeon know regardless. Please don’t try to repair it yourself. 

Hello, if you have only a tiny open wound from a single stitch coming out and it is not enlarging, you could apply antiseptic ointment and sterile gauze to prevent it from getting infected.  I would recommend contacting your surgeon to discuss whether this could be left to heal on its own or if they suggest seeking medical attention locally.