I can't straighten my leg or my foot. What is going on?

Many possibilities. You may have a nerve, soft tissue or bony injury. Have it evaluated by a professional.
Could be your back. Did you injure yourself in anyway? Did you start a new exercise regime? Are you overweight? Do you smoke? You need to see your physician for a proper work up . Your condition could be anything. Whatever you have it may be coming from your back because it entails both the leg and he foot. You may want to see a specialist for this. It may be as simple as sciatica or something more complex.