Dead screen, but phone still vibrates/powers on; what's wrong?

Although you can't guarantee a remote diagnosis 100%, there's a very high probability that the screen has simply failed and needs to be replaced. OLED screens are actually very delicate and it doesn't take much to kill them. Water damage, being dropped, or even being bent by sitting on them while they're in your pocket can all damage the display, and of course it's also possible the electronics just plain failed on you.

In any event, given that the phone is showing every indication that everything else is working, it would be well worth your time to replace the screen. Most vendors will allow you to return a display that hasn't been installed so you can buy it first then just plug it in without installing it and verify that it comes on and works before you finish tearing the phone apart to replace it. If it doesn't work at that point you can pack it back up and return it for a refund, otherwise you'd continue with the repair and there you go, Bob's yer uncle!

With Samsung phones there are two ways to replace the screen. The official/recommended/suggested method is to replace the screen and frame as a single unit; that way there are no concerns about getting the new screen securely glued to the frame since it comes that way. The downside is that you basically have to gut the entire phone and move all the parts to the new frame. The good news about that is that Samsung actually makes it pretty straight-forward and easy to do that; unlike an iPhone where that kind of operation is very difficult.

Here's a guide showing you how to do that kind of replacement. It's in Spanish but if you use it with the Chrome browser, the automatic translation makes it quite usable even if you don't speak Spanish.

Manuales / Samsung Galaxy S10+ / Full screen with chassis - Nadie Me Llama Gallina

The other way to replace the screen is to separate the OLED from the frame and just replace that part. It's really no more complicated than the full frame method, but it does require a different set of instructions, plus you have to deal with glue and resecuring the display to the frame. You'll save a couple of bucks on the part, but probably make up for that with having to buy glue. Here's the iFixit guide to that repair.

Samsung Galaxy S10+ Screen Replacement - iFixit Repair Guide

One note myself and others have run across in repairing Samsung phones is to be very careful with the rear glass. It's very thin and easy to break, and is glued down very well, so follow the instructions, use lots of heat and work slowly and carefully to avoid cracking it like myself and a few others have done. They're not expensive and easy to get, but why replace it if you don't have to?

Ah, I did think of one other possibility as the source of the problem. There's a proximity sensor built into the phone, usually near the top of the display, that senses when you put the phone up to your face to talk on the phone. When that happens, it blanks the display. A malfunctioning proximity sensor can be telling the phone that it's up against your face all the time and thus blanks the screen.

Note that this should only affect the screen while you're on a phone call, but it is possible for a bad sensor to mess things up all the time. Once again, this would require board level repair expertise, something most of us aren't equipped for, but for completeness I wanted to add this.