@PangaDayWalker games always show up by default, as long as a supported extension is used. There are options to hide files which is useful in some cases. Asumming you set up your recalbox yourself, someone might have hide games by mistake.
I got the same problem! Everything worked fine,I played mame, md, snes and c64 games without problem, then I didn't use my system for 2 weeks (pi3b+ with roms in an usb stick inside a pandora case) and now I can't start any game, and the system even does not shut off, but freezes (see the pic)0_1582914932349_IMG_20200228_185110_resized_20200228_073310000.jpg