You will need to get a voltmeter. Gain access to the rear of the headlight where the bulb goes in. Check on high beams which has power and which doesn't. Switch to low and find which one loses power. That one is high beams power. The other one is ground. The third one (that you haven't checked) is low beams power. Check it now (with switch on low beams). If it has power than there is a bulb issue - either bad/ incompatible bulb, or a bulb socket issue (is the socket clean)?
I would suspect that there won't be power. That means either there's another fuse (sometimes there is a box in the compartment and one under the hood). Check around the battery for corroded wires - sometimes there's a fusible link blown or corroded from battery acid,
Finally, how does the light switch feel? Crunchy? Sloppy? Either one might indicate a new lever is in order.
Hope that helps!