Suddenly found a wound on my hamster; what happened, please tell me.

A cheek wound may be trauma or pouch bacteria causing ulceration; check the mouth. Any wound needs timely vet care; if unavailable, shave around it to avoid infection and keep a cone during treatment. Clean blood and dirt with saline, expel pus with gauze, disinfect with povidone iodine, and if bleeding stops, apply a little chlortetracycline eye ointment, Yunnan Baiyao tincture, or powder. If no drying or improvement after care, see a professional vet.