My Plants are dying and I don’t know why

[Forest]

I am not the most experienced gardener in the world but I have had some success in the past. However, for some reason, all my plants seem to be turning brown and withering before my eyes. I have indoor plants that I move outdoors when the weather permits and it’s those plants that have taken a turn for the worse. I’m wondering if I might have damaged them by taking them outside too soon before they were ready to face the great outdoors. Something new I tried as well, which might be the culprit, is I changed the brand of plant food I had been using. Would that have this negative effect on my plants? I don’t over water them and generally allow the rain to do most of my watering for me. In times of dry spells, I will water them and once a month I give them a dose of plant food. I hope you have some suggestions as I would hate to see these plants die because of something foolish I’ve done to them.