Propane grills have a safety feature in the regulator to prevent dumping tank if hose breaks. Try turning all burners off, lp valve off. Wait a second and open lp valve then burner valve as this gives the regulator the back pressure it looks for.
I bought a new grill and after a while had trouble with the regulator jamming up. I called Weber and they told me about the above feature and resetting it. Still had problems so i called again and they sent me a new regulator/hose assembly. Works great now.