What Causes Weakness In The Biceps?

Hi, my bicep started feeling weak about 3 weeks ago, and got progressively weaker over the next 2 weeks. Then, about a week ago I was lifting weights, and it felt the weakest it had ever been and after I finished my workout, I realized I could barely pick up anything. There is no pain in the bicep, it's just very weak. I have stopped weightlifting for the past week. It has gotten a little bit better over the past week, but is still far from my normal strength. What is wrong with it, and how long till I can regain my full strength in my bicep?