My left leg gave away this morning when I tried to get out of bed- rest of the day the back of my knee kinda hurt a littl when walking-- what happened?

Knee cap? It was likely from your knee cap getting dislocated. Consult an orthopedic surgeon if this problem recurs.
Sprain. If you don't recall a specific injury then it's possible that you sprained a muscle or ligament with some sort of an awkward move. The injury sounds mild and rest, ice and if you can take them, nsaids like Ibuprofen or naproxen, can hasten recovery. See your md if symptoms persist or worsen.