Why do LED lights with a dimmer stay dim and won't go bright again?
There is a fight going on between your dimmer and your lights, and the dimmer lost.

One of the (too many) bits of electronics in your dimmer died.

Dimming is not as easy as it seems it should be, well, dimming without huge size, weight and heat.

In dimming led's, you are turning them on and off, fast, really fast, hundreds (or more) times a second fast.

For each of the on/off cycle, there are parts having to do work in the dimmer. Everytime electronics turn on, they make heat. Heat is the fastest way to kill electronics.

Your dimmer likely died in its really dim setting, and simply is not going to change.

Dimmer is gone - wants to be replaced