So I've had the S21+ since February and it's been working great, fast forward to July 11th and out of nowhere, I was checking my messages and put down my phone to drink some tea, the phone just blacks out. Now I tried resetting the phone by holding the power and volume button. It would vibrate as if it's doing it, but the screen remains black. I next tried the charger method to no avail. Now I believe it's a hardware problem, but I wanted to get a second opinion before I fully believe it's the hardware.
the display is probably dead. These sort of things are very random and you tow just drew the short straw if the phone was never dropped or bumped into any hard corners.
take the phone to UBreakIFix and have them replace the screen for you