The most likely explanation is that there is no chlorine in the water, at all. Use pool math to calculate how much liquid chlorine to add to your pool to raise FC a couple points, circulate well for a half an hour. Retest, see what happens.
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It's been hot lately, your pool could have lost all its chlorine to the sun. Do you have one of the recommended test kits? If you do a full suite of tests, and post the results here, a TFP expert can guide you about what to do. The CYA test would be especially important, as the CYA level is critical in determining how much chlorine to add, and how long it is going to stay in your water...