As Jason says, use pH Up to get your pH and TA up. An analysis of what may have happened is below (which you can ignore if not interested).
Usually this situation occurs when one is using Trichlor pucks since they are very acidic and one isn't watching the pH so it drops as does the TA until it gets near zero and "crashes" at which point the pH drops precipitously. The following, for example, shows what happens using Trichlor weekly at a 2 ppm FC per day chlorine usage if one does not adjust the pH and TA regularly (this is around 2-1/2 3" tablets per week in 10,000 gallons). In the analysis below, I ignore the outgassing of carbon dioxide which would tend to keep the pH up higher until the TA got close to zero at which point the pH would crash (drop quickly) even faster than shown below so one really needs to be checking both pH and TA regularly when using Trichlor and adjust accordingly.
.. TA ..... pH ..... CYA
120.0 ... 7.5 ..... 30.0
110.1 ... 7.08 ... 38.5
100.2 ... 6.83 ... 47.0
. 90.4 ... 6.65 ... 55.5
. 80.5 ... 6.50 ... 64.0
. 70.6 ... 6.36 ... 72.5
. 60.7 ... 6.22 ... 81.0
. 50.8 ... 6.08 ... 89.5
. 41.0 ... 5.94 ... 98.0
. 31.1 ... 5.77 ... 106.5
. 21.2 ... 5.57 ... 115.0
. 11.3 ... 5.26 ... 123.4
... 1.6 ... 4.61 ... 101.9
. -7.6 ... 3.79 ... 140.4
The above is over a 3 month period, assuming the chlorine usage rate of 2 ppm FC per day (and yes, the last result above has a negative TA, though the TA test would just show that as zero turning red immediately). If one were to add only pH Up to compensate for the pH and TA, then they could overshoot the TA. I would use an amount of pH Up that was no more than needed to compensate for the TA which would be 20 cups of pH Up per 10,000 gallons, though a little less than this amount might be needed if the pH recovers (so use less, wait an hour with circulation, retest, add some more, etc.). If the pH still needs to be raised some more, but the TA is where you want, then either aerate the water (if the pH isn't too low) or add 20 Mule Team Borax.
After you get things in balance, look at the
Pool School to learn how to manage your pool without using only Trichlor pucks. It's not just that they are acidic, since you could more regularly add chemicals to compensate for that, but rather that they increase the Cyanuric Acid (aka stabilizer or conditioner) level in the pool and that makes the chlorine less effective. You can easily get algae blooms in that situation unless you use a strong algicide.
Richard
