Can something like missing one period trigger telogen effluvium, or does it have to been a major shock? (crash diet, severe stress, etc)

Telogen effluvium. Missing one period should not trigger telegenic effluvium by itself. However a shock to the system could lead to both, such as marked weight loss and excessive exercise in women. If none of these occurred see your gynecologist or dermatologist. Also this can occur as an adverse reaction to a medication.