More detail would be nice, but if this happened all of a sudden then I'm betting that if you use a drop away rest then the cord slipped up on the cable and the rest isn't coming up all the way. You can check this by drawing and holding while somebody tries raising the rest manually. Another thing that has happened to me one time was that a limb bolt loosened up as I was shooting and the bow started shooting about 8" low at 20 yards so check this.
Could be your nocking point or loop moved up the string and or the peep slid down. It only takes about 1/8" difference to make the arrow move a foot at 20 yards. Although it's never happened to me before I recently had issues with the peep sliding down, as much as 3/8" although it was/is served above and below. Apparently since I installed it I have added some twists to the string and the twists are tighter above the peep now which forces it to slide down. This is why it is important to, once the bow is tuned, measure these things with some reference on the bow. I like to measure down to the peep and nocking point from the top axle. Then if something moved a quick check can indicate the problem without having to tear my hair out (what little is left).
OK, another possibility. Broken strands in the string or a cable, causing the nocking point to change and/or the rest to not raise up all the way. Good indicator of this would be if the peep suddenly started turning.
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