CYA disappeared over winter?

I'm opening my pool. All going well, added enough beach to clear it up (~12 gal?). pH about 7.5, TA about 140 (a little high, but not too concerned). Basically 0 ppm chlorine at the moment (wife is bringing bleach).

I went to test my CYA using the K-2006. I can't get any reading to speak of! Last fall I was reading 100+ ppm - enough to make me think maybe I should partially drain and refill the pool. Now nothing?

Any suggestions? I don't want to start throwing chemicals at it, particularly not CYA. The test kit is probably about a year old, but I wouldn't have thought it would go bad that quickly (it was stored in the basement over the winter). I can certainly go to the pool store this week for a water test...

Barring further insights, I'll plan to bump the chlorine to about 10 ppm tonight, check it tomorrow morning and then again tomorrow night. If it is stable overnight but falls off with sunshine maybe I'll believe the 0 ppm CYA reading...

When a pool is "let go" over the winter with no chlorine in it, then bacteria can grow and convert CYA into carbon dioxide and either ammonia (if you're unlucky) or nitrogen gas (if you're lucky). So first see if the chlorine holds. If instead it gets almost immediately consumed and you measure CC instead of FC, then you have ammonia and that can take a LOT of chlorine to get rid of. You can get an ammonia test kit to find out how much where you'll need at least 8-10x the ammonia level as FC to get rid of it. If instead you measure FC and not CC, then yes you can see how quickly the FC drops. If in full sun it drops about in half every hour, then you have no CYA in the water. If the FC holds in sunlight, then your test kit may have gone bad though that's unlikely since melamine which is in the kit doesn't degrade very quickly.

Thanks for the help, chem geek.
I brought it up to about 25 ppm last night (~9pm) - I overshot :-(
At 7am it was 15 ppm FC, no CC that I could read.
9am : 13 ppm FC
1:30pm : 2 ppm FC

I've tested again and see basically no CYA in my test.

I took a water sample to the pool store about 2pm. They concur with 0.6 ppm FC, CYA of 32 ppm. The pool store, of course, wanted to sell me phosphate remover (they tested at 500 ppb phosphates), which I declined.

According to the "Best Guess Guide", the behavior I see is more like the "nearly zero" CYA than the pool store's report of 32 ppm. 32 ppm feels like a reasonable target for CYA...

I did buy 1.75 lbs of CYA. According to the label, 1.75 lbs of CYA should raise the level in my 20K gal pool by about 10 ppm. I went ahead and added the 1.75 lb, plus more bleach to keep FC above zero.

So, questions:
1) I assume I did good by ignoring the phosphate remover, right?

2) Since I read basically zero CYA and the pool store reads 32 ppm, adding just one container (1.75 lb) CYA seems warranted and safe. Was that reasonable, or should I add more (or, maybe, less)?
2a) Is there a consensus on a good target for CYA? This is an outside pool, with moderate/low sun exposure.

3) Chem Geek mentions that this can happen to a pool that is "let go with no chlorine over the winter". I honestly don't know how to maintain chlorine levels over the winter... I start adding a little chlorine early in the spring, then open the pool later (about now). It is always green, sometimes I can't see the drain at the deep end. I vacuum, adjust pH, then start adding bleach, vacuum, repeat..., until it clears up (usually 2-4 days). Am I doing this in a reasonable fashion?

So your overnight loss is slowing down but to prevent loss during the day you need to get CYA in the water. You can either add pure CYA or can use Dichlor granulated chlorine to add both chlorine and CYA at the same time. Keep your FC level elevated to be around 40% of the CYA level until your overnight FC loss lowers to around 1 ppm or less.

Yes, you did well ignoring the phosphate remover. That would not have helped.

A reasonable target CYA is around 50 ppm in a pool exposed to sunlight. Some pools need more (but not more than 80 ppm) while some are OK with less.

To avoid the pool turning green over the winter and to lessen the chance of bacterial breakdown of CYA, you should close the pool as late as possible with the pool water cooler than 50ºF and at that time you can shock with chlorine and also add Polyquat 60 algaecide. Then in the spring you want to open the pool as early as possible before the water gets any warmer than 50ºF.