Sounds like your roots froze.

As for over-wintering strawberries, the usual trick is to cover them with 5 or 6 inches of straw and let it snow. They will survive this way, usually without a problem.

The "stolons" or runners actually need a certain amount of "cold hours" (usually around 160) per year to assure fruiting in the following season. Cold hours are in the neighborhood of 35 to 40 degrees and of course, do not include a "hard freeze".

I'm currently testing a product that is supposed to protect plants down to 27 degrees F. I'll post the results as soon as I have some.

Oops. I kinda missed the boat on your question; somehow I overlooked the "hydro" part.
On that note, you'll have to figure out a way to keep the air temperature warmer. This will probably mean growing in a heated greenhouse.

Of course, you could do what the rest of us do and merely buy new "pre-treated" bare-root stock each year. That way, every year is a "first-year" crop and the fruit production is its highest. Second and third-year productions are far less than the first year's.

If you do the bare-root thing, be sure to order by late October or November or you may be out of luck. They go fast and when they're gone; they're gone. If you can hook up with a commercial grower, you might be able to buy for cheaper. The commercial crew buys them by the pallet. That's about 12,000 individual plants. They're REALLY cheap when you buy them like that!

"Pre-conditioned" by the way means the plants have had the necessary amount of "cold hours" before they are shipped. So, if you want to propagate from your own runners, you'll have to make sure you give the new starts about 160 hours of chill time or they won't fruit!

Hope that sets things straighter! :O)