This Deadspin article's title says it all:
After reading that article, we clicked over to Daily Norseman to find out what's going on (LINK)
"During the Vikings' practice on Tuesday, Teddy Bridgewater went back to pass and then went down without contact. He went to the ground, holding his left knee and appeared to be in a great deal of pain.
The Vikings called practice off about 20 minutes early. Bridgewater had players surrounding him, as well as players cursing about what happened to their quarterback."
The ambulance came and picked him up, and that's all we know right now.