What happens to a human if suddenly loses all spinal fluid?

Immediate effects - Extreme pain; the spinal fluid is is also what supports and cushions the brain. The symptoms would likely be similar to meningitis.

Long term, it depends on other factors I think. The brain would be far more sensitive to movement or impact injury and any movement would be excruciatingly painful.

The person would also likely be far more sensitive to infection. The membrane that retains the fluid also helps isolate and protect the brain and spinal cord, so whatever allowed the fluid out may allow microorganisms in.

There would be cell damage to the nervous system and brain. The person could die, pretty quickly, or suffer partial paralysis or mental debilitation. Very unlikely that there would be no effects at all. Hard to say in detail because there isn't really any way to make the hypothetical event happen (at least not without some other major trauma that would itself be potentially fatal).

That's fatal!