You are either out of gas or your excess flow valve tripped. First check for gas in your cylinder. Typically a full cylinder weighs about 34 pounds and about 17-18 when it's empty. You can weigh it on a household scale. Or better yet, use a gauge.
If you have gas, you may have tripped the excess flow valve. So, turn off the control knob, disconnect the regulator, wait 30 seconds and reconnect the regulator. Turn on the tank valve slowly, then light grill according to lighting instructions.
If flames are still low, repeat this process up to three times. If this process fails the excess flow valve may have become stuck in the regulator and the regulator will need to be replaced.