What breed of dog?
Did you see any trauma occur?
How long has this been going on?
This sounds like an injury to his back, more specifically, a problem with an interverteral disc. This is very common in dogs like the dachshund, but I've seen this problem in many different breeds as well. When the dog injures a disc in the back, the disc impinges on the spinal cord, and cuts off the nerve supply to the back legs (or back and front legs if the injury is in the neck). That may be why your dog cannot get up on his back legs. This is a serious problem which should be addressed by a vet ASAP. This means going to an ER since it is late saturday. If this is the problem, only prompt attention will help him recover. This longer you wait, the less chance he has of recovering. Unfortunately, this could me surgery, depending on how severe the problem is.
Here is some info on this problem, with pictures of the back: http://www.gcvs.com/surgery/interver.htm
Another possible reason for these symptoms is severe arthritis. If your dog was very active today, and is now stiff and sore, he may have trouble getting up on his legs. There could also be some other sort of injury that is painful and making him not want to walk.
The best thing for you to do is take your dog to the vet. Once they examine him, they should be able to tell you what specifically is wrong and give you some treatment options.
Let me know if you have other questions.