Put the multimeter on volts DC nearest range over 12v. Measure voltage accross battery terminals with the engine stopped. Put the meter on a higher DC volts range,start the engine, measure the voltage accross the battery terminals. Voltage should go up from 12v ish to 13v ish maybe 14. If it shoots up high, the regulator rectifier is duff and will probably have cooked the battery. No increase could be the alternator or the rectifier or a break in the wiring. Check the battery for fluid. If it's low , top it up with distilled water. Melted Freezer ice will be ok as it has been de ionised. If the battery has been cooked it will not hold a charge after charging and will need replacing. All this will be in a Haynes manual with drawings, photos and specific information for your bike.