it always helps to tell us where you are located. There are hardly any garden questions that can be discussed without regard to location/ climate.
Watermelons, as you might guess from the name, like water. Rain by itself should not have harmed it, unless the drainage is very
bad and there was a puddle of water staying around the roots.
Have you had a frost/freeze? Watermelons are very tender and a freeze would wipe it out. Otherwise check the base of the stem, just above where it comes out of the ground. If you find a hole or chewed up looking area, with some orange-ish, sawdusty looking stuff around it (maybe on the ground below it), something like this:
http://3.bp.blogspot.com/-5xBLfl06qew/T ... MG3071.JPG
it was attacked by squash vine borer.
As the name suggests, the borer usually attacks squashes and pumpkins, but if there are none in your garden, it might go for the watermelon.