Your spa possibly has an air lock.This is a very common issue that generally occurs after draining and refilling a tub.An Air lock happens when the hot tub is being filled with water. It is possible for both ends of a plumbing system to simultaneously fill up with water at both the suction end and the return end.  This sometimes leaves an air lock in the plumbing. This can cause both heater and pump problems.  It is important to notethat properly filling your hot tub is the best method of preventing an air lock. To properly fill your tub, make sure that you insert your hose into the filter standpipe while filling your tub. The standpipe is the area left vacant after you remove your filter. The standpipe is the one with an opening at the top of it. Only one standpipe is like this in units with multiple filters in the compartment.If you believe you have an air lock, there is one simple step that you can try prior to calling for service. With the filter removed and power off, insert garden hose all the way down into the filter standpipe and turn hose pressure to high. With water running, turn the power back on to the tub. This may assist the tub in burping itself and removing the air trapped inside. On some models, it may help to activate the jets. If an air lock has occurred the tub will not heat.