How cold is it there?
The SmarTire sensors use Lithium batteries. If not used for a long period, Lithium batteries will show a low voltage on the initial use due to a passivating layer that builds up inside the cell whenever it sits idle. If you continue to use them, the layer will break down and the voltage will slowly recover back to normal. The key to keeping lithium batteries working good, is to use them often.
Try going for a few more rides and see if they don't start working again.
You also may need to reprogram the receiver. Turn off the key, HOLD in the button on the receiver, then turn on the key and continue to hold the button for about 10 sec until the display flashes. Then spin the rear tire to lock in the rear sensor code. The display should then flash to the front sensor. Next spin the front wheel to lock in it's code. Once both codes are locked in, you HAVE TO PUSH THE BUTTON AGAIN to lock in the codes. If you don't do this last step the codes will be lost when you turn off the key. Once you have pushed in the button to save the codes, you can power off the bike.