What are the ingredients in HTH pool shock? In your pool, each cup will add about 0.75ppm of chlorine. I'd add about a half a gallon (2 quarts) of plain, household 6% bleach. That should take you up to about 6ppm. Then, this evening, test it and add however much bleach you need to get back up to about 6ppm.

Did you add any stabilizer yet? Trichlor pucks have stabilizer in them, but it will take a little time for the stabilizer level to come up. In the meantime, I'd suggest testing morning and evening and each time, add a little bleach. Until you have a stabilizer reading, your chlorine will be lost pretty quickly to the sun.