So, my mom has had a Galaxy S5 phone for awhile, but I went into her room today and found her old Galaxy S5. She claims it's not working properly, so I offered to try and fix it. I spent a few minutes messing with the phone and I noticed that I could not get the screen to turn on, but the phone vibrates when I plug it in, and my PC makes the device connected sound. I'd like to know how to reset the phone to factory settings, but also flash a custom ROM onto it. (In this case, I want to flash Resurrection Remix) My mom will be okay with this since she doesn't use the phone I'm trying to work with anymore. The phone is of the AT&T variant.
Edit: I asked my mom about the screen problem. She stated that the top of the screen started to dim and within 48 hours the screen wouldn't turn on at all.