General things to look for if your bike won't start
There are only 3 major systems on any internal combustion engine.
That said, the 2 things (in order) to check for are water in the float bowls and that the spark plugs are generating spark. These are extremely simple tasks to do and are a great introduction to working on your own bike (if you have even the slightest desire to do so). Water in the fuel is nearly the ONLY thing that will cause the bike to work fine one day and then suddenly not work the next. You may have, for instance, filled up with a bad tank right before going home, and had cups of water go straight into the carbs. If you don't want to do your own work, then try making sure all the integrated safety switches are tripped, meaning the kickstand is up, bike is in neutral, and clutch lever is pulled in.
If those things are done and you still don't get any cranking, then buy/borrow/steal a multimeter and check the battery voltage. Then, with the meter still hooked up, try to start the bike. If voltage reads 12.xxx VDC and drops to below 10VDC, you need a new battery.
If the bike IS cranking over, and it hasn't run in awhile, clean the carbs, check/replace the spark plugs, and check/adjust the valves. Your bike won't start/run without one or all of these things being done.