Please take your dog to the vet right away. I am worried your dog may be bleeding internally. Some older dogs will develop tumors on the spleen that rupture and start bleeding suddenly. It is also possible that your dog got into a toxin of some kind. Check his side. If he has a swollen side he may have a twisted stomach. This is a surgical emergency.
You are correct a dog in good health that suddenly wont get up is in trouble.
Seek attention for him now.
Melanie Crovo, DVM