Why is my dog having trouble standing?

I have a bet hound and she will be 2 years old in December. She was fine yesterday, her normal self. We have three kids and when they go out there she greets them and wants to play, or even when we are outside she is always following us around. Today though she is just laying under the deck in a hole she dug (most likely because it has been around 100 degrees lately). I finally coaxed her out with some food and she acts like she can't hardly move her legs. I have gotten her to stand up and walk just a little bit, but she will just sit back down (which is very unusual for her). She is normally an active dog. There doesn't seem to be any pain either because I have felt all over and squeezed her legs and hind area looking for a tender spot, but there was nothing. She ate some dog food I gave her and she is drinking just fine. Was just wondering if this has ever happened to anyone elses dog?