If your gas cylinder is empty, refill it.

If the problem is an unusually slow burn, you may have triggered the safety valve (Disk shaped thing attached to supply near cylinder).

Follow proper lighting procedure.
1 Switch all regulators off. Tank, then burner to release any pressure in supply line.
2 Open regulator at cylinder fully. Not a half turn or two. Wide open.
3 Open regulator in front of barbecue and light up.

Make sure the propaine is not to low and the burner orafice is clean