Suzuki Bandit 400 suddenly no longer starts

Hello, I’m new here hot Hannes and I’m 20 years old. Now to my problem where I just don’t know any more. I’ve been driving a Suzuki Bandit 400 with 34 hp with about 15000km. So far so good, it really ran perfectly over 1000km. Always stood up to 5 days dry in a garage. Suddenly, when I wanted to drive after a week, you didn’t jump any more. The starter spins and turns… 2 batteries I completely charged and torpedoed.. Nix.. With Choke/without Choke, with gas without gas tried, everything was brought nothing. One could always hear a quiet, “grumble” from the exhaust, as if she wants to come, but it does nothing. Then all spark plugs out and nailed, so that possibly fuel flies, brand new spark plugs in, however the same as before only that there were even a few false ignitions in the form of loud banging out of the exhaust. Ignition spark is there on all four plugs, candles are also wet after the orgling, so also fuel arrives. Air filter is also fine. According to the dealer, carburettor has enjoyed an ultrasonic bath, which is also plausible due to the perfect and quiet running properties. I really don’t know any more, hope that someone may know about it and can help me. Especially the sudden no longer start without what I did is funny. Thank you in advance