DIAGNOSIS You have spark but no gas. The most likely cause is an empty propane tank. Second most likely is the tank valve isn't fully open.
FIX First, check your propane tank level. Swap in a known full tank. If you have gas, ensure the tank valve is opened fully. Turn the knob counter-clockwise until it stops. Then try lighting again.
IF THAT DIDN'T WORK Check for spider webs or debris in the burner tubes. Insects love to nest there. Turn gas off. Remove the cooking grates and flavorizer bars. Visually inspect the burner tube holes for blockages. Clear them with a pipe cleaner or compressed air.
HEADS UP Always turn the control knob to OFF and wait five minutes before a re-light attempt after a failed ignition. This lets accumulated gas clear.