I dropped my 1995 ZX6R recently while it was standing and running in my drive. After picking it up and starting it I was unable to build oil pressure. I rang several mechanics with my concerns and found that this is a known problem with these models. Apparently air becomes trapped in the oil lines and remedy is to top up the oil to the correct max position and then lay the bike as far as possible in the opposite direction and then stand it up again. This should get the oil line full again and recover the pressure.
I have noticed that it is difficult to build pressure after an oil change and the recommendation is to drain the old oil and refill it as quickly as possible to avoid air trapping.