I would shut the gas valve off and wait for five mins then relight the pilot and see if you can get the burnerto come on. Try this two or three times and if you do not succede then I would replace the gas valve. Thank you

Did someone possibly turn the control knob from ON position to the Pilot Position. If that's the case, the burner won't come on.

Another thing that might cause your problem is low gas pressure. If you have LP Gas, the regulator on the gas tank may be frozen or the tank may be almost empty.. This happens frequently when temperatures remain well below freezing. To test this theory, soak a bath towel in hot water, ring it out and quickly place it around the LP tank's regulator. This will thaw the regulator and allow gas to flow smoothly to your heater. Remove the wet towel from the regulator once it has thawed.

If the freezing continues to occur, wrap insulation around and inside the bonnet of the tank. You may also want to contact your propane provider about this. As they may need to add some ethanol to your tank.