You poor thing, this happened to me about 10 years ago, I still feel upset we were out and a bit later back than we expected and found one guinea dead on the lawn and 2 were missing and we never found any trace. The children were hysterical and I cried on and off for days, don't feel bad about being upset. Our neighbours lost a rabbit the same summer the fox managed to get into the hutch. I have 10 guineas and 2 rabbits now, but live in an area with much smaller gardens and not so many foxes. I do also cover my hutches at night and fasten the covers down with tarpaulin clips and my daytime run is on the patio with a double tied string holding it shut.
Foxes are a nightmare, my friend's husband breeds guineas in the same area where I used to live and only puts them out now when he is actually in the garden, he has seen foxes come and try and get into the run in broad daylight even when he is there.