Your petcock picks up fuel from one of two points, one higher than the other inside the tank. In the "ON" position it picks up fuel from the higher point until that fuel is exausted, then in the "reserve" it picks up fuel from the lower portion until the tank is empty.
If you leave it in the "reserve" position you'll have no advance warning that you're running short on fuel because the tank will run til it's fully empty.
Your tank only holds 1.8 gallons, so you were well into reserve territory when you left, so probably there is nothing wrong with the petcock.
If with a full tank it still won't feed fuel in the "on" position there are two possibilties with your bike: One is that the petcock is clogged on the "on" side orifice inside the petcock. The other is that the standoff tube filter inside the tank is varnished and/or clogged.
There is also a third (remote) possibility that some prior owner put a petcock from another model Yamaha on there. Almost all of them are identical except for the length of the standoff tube for the "on" position, and if they chose one that was too long it would hit reserve prematurely.
The other function of the petcock is the "off" position. Since our bikes are a "gravity feed" arrangement in the event of a worn float seat or if the float gets knocked out adjustment it will flood the carb, then the airbox and eventually fill the crankcase with gas. Any time you park the bike overnight or for any extended period you should turn the petcock "off".