There's not much to go wrong with the propane system so troubleshooting should be pretty straight-forward. Here's what I would check:
The propane solenoid...if you don't hear a distinct click when you press the solenoid button on your propane detector, you may have an electrical issue. I'd check the fuse for the propane solenoid. If the fuse is good and you've confirmed the solenoid has power, it's also possible that the solenoid could be stuck open if it hasn't been used for a while. Otherwise:
The obvious...make sure that there is propane in the tank and it's connected. There's a safety device in the tank valve that will stop the propane flow if it is excessive. That safety can get activated if the valve is opened too fast. I reset mine by closing the tank valve and disconnecting the hose from the valve to release the pressure. Connect the hose up and open the valve slowly.
Another less-likely possibility is that there is an obstruction in your gas line; if so, probably near the burner.