If the pain is that bad, I do highly recommend a doctor, as badly healed injuries in something as mobile as a wrist can be bad news. As regards stretching, I advise looking at Aikido wrist stretches. They cover a fairly wide range of movement and are easy to adjust to your current flexibility.
On a side note, I had something like this happen to me a few years back. I let my wrist relax too much while hitting the punching bag. I made a full recovery and it turned out to be nothing worse than a sprain. I wore a wrist guard for a while, and found that pushups were much less painful when I did them on my fists, as it involves a straight wrist.