We have no easy answer but a couple of ideas. It might be rabbits; you do not say what the fence is made of, and they can squeeze through very small places. The apparent almost-overnight disappearance is especially puzzling but we’ll take a stab at this: Is your soil too rich? That is, could the pH be off? is it holding water (is it too tick/rich/heavy to drain well) and drowning the roots? Compost is good but maybe, just maybe the mixture lacks essential nutrients. You could do a soil test. Cooperative extensions perform the most thorough tests; find the service nearest you here: https://www.almanac.com/content/raised-bed-gardens-and-small-plots#comment-85906
When you contact the service, ask them if they have any other ideas, based on your location.
We have no easy answer but a couple of ideas. It might be rabbits; you do not say what the fence is made of, and they can squeeze through very small places. The apparent almost-overnight disappearance is especially puzzling but we’ll take a stab at this: Is your soil too rich? That is, could the pH be off? is it holding water (is it too tick/rich/heavy to drain well) and drowning the roots? Compost is good but maybe, just maybe the mixture lacks essential nutrients. You could do a soil test. Cooperative extensions perform the most thorough tests; find the service nearest you here: https://www.almanac.com/content/raised-bed-gardens-and-small-plots#comment-85906
When you contact the service, ask them if they have any other ideas, based on your location.
We hope this helps!