The safety "flow" check valve in the propane tank has tripped.
Open the valve on the propane tank very slowly. This usually prevents the safety valve from tripping.
When mine does this I turn off the grill close the propane tank then reopen it slowly and relight the grill usually works every time.
If that doesn't work turn off grill close propane tank valve disconnet hose from tank. Turn on grill but do not light let the propane bleed off. Close grill valve reconnect hose, open tank valve slowly, open grill valve and light. Should work.
Steve