Gsxr 400 bike wont start

I would say that the carbs have to come off for cleaning, the float needle is jamming either by dirt or wear.
You may need carby kit(s).

Sounds rare that all carbs are flooding.

The only way not to let this repeat itself is to lock your fuel-valve everytime the bike is not in use.
There's supposed to be a sack of minor tools sold with the bike. The tools are easily reached by opening the little compartment area in the tail section of the bike.
As you might have been well aware, a bike is essentially different from a car that has thousands of parts attached to it. Your bike has a simple mechanical fuel-valve, not a pump as in cars. So whenever the fuel-valve is 'on' you have the fuel going all the way down to the carburettors, and the only thing regulating the fuel flow is just the simple 'float' in the carburettor.
If you experience starting difficulty in the future, you might want to replace those carburettor floats, because they have suffered as a result of you not shutting-off your fuel-valve each time you park your bike.

You will have to remove the carbs from the bike and disasemble them and clean them. You will need to get a manuel and it will step you threw the prosess step by step.