You're sure the heater is broken? Did you make certain your filters were clean?
I know some might not like this way of checking, but as long as there is not debris in the tub, you will be okay for a little while.
Take out the filter with the gray cap. Power down the spa for about 30 seconds then restore power. See if the heater starts to work. IF it does, then you know you have dirty filters and it is time to clean and rotate them.
Not sure about the Michigan weather vs keeping warm till Monday. I'd ask your dealer on that one. I assume that since we're only in the second day of 2009 that your spa is still under warranty so if the heater is bad, you won't have to incur the cost of replacement.