broken paw? or sprained?

My dog was running around the park, and she runs at full speed. She runs up and down hills, over logs, puddles, anything you can imagine. So when she walked back home she was walking fine, she went up a flight of stairs fine... until a few hours later she tried to get up and was already limping. Now she can't put any pressure on her front paw, well she tries but it obviously seems to hurt her. We're going to take her to the vet but we have to wait until tomorrow since it's closed... Would she have been able to walk back home or up the stairs if her paw was broken?