If your grill has been sitting idle for months, after you dust it off or uncover it, it's a good idea to take a close look at all of its components to make sure everything is in proper working order. Examine hoses for any cracks or damage. If a hose seems compromised in any way, replace it.
Inspect burners and their gas tubes and make sure they aren't worn out, rusty or blocked by debris or spider webs. If gas tubes have worn spots, replace them. If they are blocked, use a pipe cleaner, toothpick, paper clip or other slender object to clear them. Check knobs and other controls to make sure they are not damaged or non-working. Tighten any loose screws or bolts. If your propane tank is empty, refill it at an authorized propane supplier.
To avoid serious injury, be sure the propane tank is disconnected from the grill before doing any kind of maintenance. Read your BBQ grill's instruction manual for particular safety issues.