You've got three wires at the bulb. One is live for dip beam, one live for main beam the third is an earth which is shared.
If the earth is faulty your light will usually only glow cos it earths thru the other circuit doubling the resistance

Easy test. Get a simple test light, a 12v bulb with two bits of wire will do, connect one end to a known good earth like something metal on the engine. Stick the other end in the lamp connection. Turn dip beam on, does it light up? No? Try the other two terminals. One lights up that's dip live! If none light up you either have no power there or your test light isn't working! Stick it on the main power cable at the back of the starter, lights up? Yes, then you have no power at the lamp! Check fuses, switches, relays and wiring! Repeat for main live, you only need to test two wires this time cos you know which one dip is! The remaining wire will be earth. A good basic test is to connect one end of your test light to a good power supply, back of starter or similar, earth the other end to check it works! Note how bright it glows, now stick the earth end into the third connection, the headlamp earth, it should glow as brightly as it did before. If it isn't as bright you have a resistance on the earth, usually corrosion. If it doesn't light the earth is either broken or you have a very high resistance. Trace the wire check for breaks and corrosion particularly where it bolts to the body!
Easy

you've just checked your lives and earths at the lamp without a diagram!
If your lives and earths are present change the bulb and check the connectors are a tight fit to the bulb they get weak with age then hot then melt the plastic holder!
The above is a good basic testing method for basic circuits only!
See how you get on. A basic cheap test light is invaluable for basic electrics. Much simpler than a multimeter. More complicated stuff you'll need one but you need to learn how they work to use one properly. So stick with the basics until you get the hang of it

lesson over
That method is how some manufactures teach you to do it. Simple and it works. W