It sounds like she could have had a seizure when she hit her head. Some dogs have seizures where they completely freeze up and become catatonic for a short period. Most likely what happened is that when she hit her head, her brain hit the top of her skull with such force that it bruised her brain.
Unfortunately head trauma can cause brain damage or hydrocephalus which can't be diagnosed from an x-ray or blood tests. Ask your vet if he did a neurological exam which will help to determine the extent of the head injury. Keep a close eye on her and if she exhibits any neurological symptoms associated with brain injury, then you will want to do further testing.
Hope she gets well soon!!
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