If you can ride without pain, ride; but take it easy. And I definitely would not recommend getting out of the saddle to climb or accelerate.

Cyclists who don't stretch can have a tendency to shorten their hamstrings. This in turn can cause all sorts of other problems, whether or not the hamstrings themselves give trouble. So as others have said, once you've fixed the immediate problem, get into a routine of stretching your hamstrings (and your quads, I'd suggest) before and after you ride.