9 1/2 times out of 10 it's a rusted up pivot point for the brake lever.
You can do a temporary fix by spraying some lube into it, but the best way is to remove the brake lever and clean and grease the pivot. For those most adventurous, install a grease nipple on the pivot point.
When mine stuck last year it was mostly gunked up around the pivot on the outside edge.
I pulled the pin from the inside and washer and working it up and down by hand sprayed WD40 or PB or such till it worked free, then I put some grease on the pivot and worked the pedal in and out while working up and down to try to get some up into the center.
I've not had any problems since.
Mine would not free up with just spay till I got it worked over to the side some where I could actually get to the pivot.
Was not hard to cleanup and fix, but I could not get the pedal all the way off, and putting the washer and key back in was a pain.
I got big fat hands LOL