The screen of Nuheat Solo keeps flashing, the floor is cold. It seems like that the whole system stop operating. Yesterday, it was fine.

I had the same problem. It is caused by a failure in the thermostat, usually brought on by a power surge. Unfortunately, this product is poorly designed, and cannot handle common power surges in homes. The only solution is to replace the thermostat unit. Nuheat has a decent warranty (3 years) so you may qualify for a free replacement.