What could cause an onset of severe muscle fatigue in a middle age adult?

Difficult question. This question needs more data. If it is after sports activity, we still need to know after how long and how much sport? This usually as a result of spine problems in general, or arterial flow promblem. If occurred suddenly with no sport , and persisted, then question is embolism to brain or ischemic problem, if resoled there is a question of tia.
Diet, Meds. Stay hydrating eat well if taking medications ask for docotr or pharmasist about possible side effects if none of the above apply, then see you md for blood work and evaluation.