Since this morning my dog started to drag himself around refusing to stand on his back legs. He is shaking and his tail is between his legs and we can't pick him up or he cries. What is wrong with him?

Sounds like a possible spinal injury, he needs vet care ASAP. Only a vet can tell you for certain,
If this is the case he will medications and treatment that only a vet can provide.
Please take him in for a check and best of luck to you and your dog.
What kind of dog is he?

Sounds like IVDD (Inter vertebral disk disease).

Your vet will be able to tell you for sure.. Or they may refer you to a specialty center with more high tech xray machines for a better diagnoses.

In the mean time.. Here is some info on IVDD.. common symptoms and how to handle and care for a dog with back pain.

Intervertebral Disk Disease (IVDD) | HealthyPet U (http://www.healthypetu.com/medical/nervous_system/dog/intervertebral_disk_disease_i.aspx)