You would have to show that the state were negligent (or unreasonably careless) in some way--such as that the mower were not properly maintained, or that the mower's operator drove it over something (like a patch of gravel) which it was unsafe to drive it over. Unless you can show fault, the state would not be liable; the state does not act as your insurer, but would only be required to pay to the extent it is at fault.