I doubt freezing had anything to do with it. More likely, it's the same thing we've been seeing so much of this season: bacterial bio-degradation of CYA (stabilizer) to ammonia, creating a HUGE algae demand.

Do you really have a 40K gallon pool? If so, we may need to look for a low cost source of chlorine; it can take 100+ ppm of chlorine to clean up the mess, and on your pool, that could be equal to 130 gallons of household bleach.

Here's where I'd recommend starting:

1. Get 4 quarts of polyquat, to keep your pool from becoming a total slime pit. (info here). If you can't find it locally, here's an Amazon link @ $18/qt:
Kem-Tek 60% polyquat

2. Do a bucket test for chlorine demand.
+ Get a CLEAN plastic bucket or garbage can, and fill it with 4 gallons of pool water (use a scale: 33 lbs + the weight of the bucket).
+ Dose with 1/8 cup of bleach, mix, and test with an OTO (yellow drops) kit (NOT: 'guess-strips' or DPD color match!!) You should see orange or brown.
+ Wait 30 minutes and retest: if there was high chlorine on the first test, and none on the 2nd test, you have confirmation of HIGH demand. If you see yellow, things aren't too bad.
+ Wait 1 hour, and retest. See if there's a difference.
+ If you've confirmed high chlorine demand, continue with 1/8 cup doses and 1 hour waiting periods until your chlorine level STAYS orange or brown.
+ Wait 4 hours, and see if there's change.
+ Let me know what you find.