Assuming that the bulb isn't burned out, the dimmer failed. It happens.

We installed simple incandescent dimmers for all of our lights when we moved into our house 25+ years ago. We've replaced every single one of them at least once. Maybe it's because we bought the cheapest one on the shelf at our local big-box store...

Replace the dimmer.

Cautionary note:

Be sure to:

• Turn the power off at the breaker

  • The easiest way to know you've got the right breaker is for the light to go out, but since the light's already out...
  • Pick up a non-contact voltage detector while you're getting the new dimmer. For $15 or less it can save your life.

• Have the new dimmer handy before removing any wiring

• When you've got the old dimmer out of the wall, but BEFORE removing any wiring:

  • Grab your cell phone & take good, clear, focused pictures of all the wiring connections.
  • Take good, clear, focused pics of any extra wiring in the back of the box

• If you're not clear on how to rewire the dimmer based on the new one's instructions and what you're seeing in the box, come ask a new question here before disconnecting anything.

  • Include all your good, clear, focused pics
  • Include the make/model of the new dimmer. A link to the mfgr site (and instruction PDF) is always appreciated

It's a lot easier to ask a "stupid" question than it is to rewire it wrong which could damage the new dimmer, the fixture/bulbs, or you.