One day our spinach was growing just fine and the next day they all looked like something had been sprayed on them, but not. The lettuce next to them was fine. The spinach in separate bed was the same way. What's up?
This sounds like dampening off fungus.
Dampening off is a fungus that lives in the soil, particularly in humid weather.
Do not plant Spinach in cold, damp soil. Soil should be well draining.