18650 battery cells died in the winter weather, can they be restored?
Unfortunately not, at least safely. Cycling the cells may help if they are still within safe voltage ranges, but is unlikely to undo any of the damage itself.

The battery chemistry itself is likely damaged. I have often stored batteries fully charged, out of convenience rather than lack of thought, and over time the resistance of the cells increases as a result. I have never had any die to that extent over a single winter, I never store them outside in such a state though.

With respect to your predicament, I would suggest cycling them, and if you have / or can obtain a lipo charger with capacity and internal resistance (as many hobby grade chargers for remote control include), you should be able to gauge the state of the batteries to see if the issue is as bad as it seems. It is possible they have usable capacity for other applications but no longer for high-current applications, like your hoverboard, due to a  large increase in internal resistance, which in-turn, causes the voltage to sag and your battery protection in the board to cut off.

In summary, you are unfortunately likely to be looking at new batteries for the hoverboard, but you may be able to make use of the cells for less demanding applications, if they are not completely destroyed.