Hi I have a 03 katana and for some reason the bike won't power on anymore...I turn the key and nothing no lights nothing. I'm not sure if it's the battery or what. What should I do to trouble shoot the problem
First thing you do is check the battery with a multi-meter (as mkaatman said). If the battery is good, it will have a reading well over 12.5vdc. If it is below this, put it on a charger and get it to 100%, which while sitting should be around 13.1vdc or more. I suspect the power level of the battery is going to be much lower than this and believe your battery is the main culprit here.
If you personally cannot figure out if the battery is good, take it somewhere like AutoZone or Advance Auto and have them load test the battery.
If the battery checks good, you need to check to ensure you don't have any corrosion on the battery leads (where the cables connect to the battery posts). If you do, this will prevent the battery from charging correctly.
If this, then, does not prove any issues, you'll need to make sure the alternator on the bike is charging correctly. This is going to be a little more tricky. If you get the battery recharged (or back to a state where the bike will start), you'll put a multimeter on the battery while it's running. The voltage should read in the 13.8vdc+, but should not be over 14.5vdc. If your initial reading is at battery voltage (~13.1vdc), but slowly decreases, this is a sign the alternator is bad and will need to be replaced. I'm not exactly sure on a bike how the charging system works (size of alternator, where it's exactly located, etc), so will leave it at that. Needless to say, if the battery is not recharging, your battery will not stand up.