This indicates a High Limit condition. The temperature protection system is reporting that the water temperature has exceeded safe limits, and has shut down the heater. As long as the HL switch is tripped, the heater will not come on, even if the water is actually cooling down. This does not necessarily mean that your water has overheated; there are other factors that can cause this particular report. 

The first thing to do is to shut off the GFCI breaker for the spa, wait 10 seconds, then turn the breaker back on. This will force a system reboot (yes, your spa is run by a little computer!) and restore all settings to factory default. If this clears up the HL report, all is well. Simply re-enter your filtration cycles and other custom settings. 

However, if the HL condition remains, or happens again within a short time, please contact your dealer for service.