If your starter is making a grinding noise when you hit the button, there could be a number of different problems. The problem could lie in the starter itself, in the starter clutch, the chain or gears which connect to the flywheel, or a damaged flywheel.
Owners of older Yamaha Viragos know what this is all about, a manufacturers defect made these bikes especially prone to noisy starter problems.
Typically this is a slowly progressing problem which gets worse until it fails, however sudden failure is also a possibility. There isn't much you can do in an emergency situation to repair a starter motor which ground to a halt. I usually replace them, but they can be rebuilt. However it is worth investigating whether your chain or idler gears has come off. You can do this by removing the starter motor cover, and see which end goes into the motor (usually left). Then remove that side engine cover, and do a visual inspection, and see if anything is out of place. If it is, do your best to set it right. Get the bike home and get your repair manual out, and repair or replace any damaged or worn parts, including the flywheel if it has damaged teeth.
You can still start the bike though without the function of the starter motor.