Iphone 12 mini screen defect

my iPhone 12 mini keeps shutting off. The only thing I can get it to is the black screen with the white apple. I went to apple yesterday and they "fixed it" but now I can't even get my phone to register with my computer. Idk what to do or how to get it back on. Ive tried all the hard resets and force resets but it won't go past the black screen. How am I supposed to Get it turned on. Ive also been trying to use my MacBook but it won't register.