Hi Valerie,
Best guess is a cracked return line pipe. If that is the case the water level will stabilize at the level of the return jets. With just a cold snap like that it would be unlikely for a completely buried pipe to freeze that quickly but if there is any part of the low lying pipe exposed then that could freeze. One section of my return piping is under a wooden walkway from the house deck to under the pool deck and this section would freeze quickly in my case. Should be no problem to let it drain and either fix or demolish in the spring. If it stops at the return level you know the problem but if it keeps on going then it's either a liner leak or possibly a leak at a low spot around the stairs. Keep an eye on it and report back.
Otherwise, relax and have a Merry Christmas.
Al
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