99.9% it's the LCD screen, I purchased my first Switch for cheap ages ago that only worked on docked mode, undocked I can hear sound when tapping on the screen. If you tilt the switch in a certain angle with a light source shining on it, you can see the display is actually showing something but no back light and you can navigate with no issues, so that tells my my digitizer was still good, as someone already mentioned the digitizer and LCD screen are separate.
I got a replacement LCD on eBay for cheap and replaced it myself, and it's been working ever since.
Here is a few things you should know and consider:
Dusty Environment:
Beware of dusty environment and grab yourself some compressed air in a can, doesn't matter how hard you try to wipe the new LCD screen and the digitizer, the static of the screens will attract dust and this will annoy the hell out of you once you put it back and there is dust under the screen.
LCD Screen:
Doesn't matter if it's original or aftermarket, the LCD is smaller than the frame where it sits in, you will have a few mm gaps all around, yes there is a few notches on the frame where the LCD will sit nice and secure but once you put the digitizer back on, the LCD screen WILL be out of alignment. You will notice when testing the screen before you put it all back, the LCD doesn't use all it's surface area, there is a small boarder, so you can imagine trying to align everything up when you put everything back together. For some this wont be an issue but for others (like me), just noticing the screen is not centered and not aligned can get on your nerves pretty quickly. I assume from factory they have some sort of jig or template they use to install the LCD. The digitizer which is the front screen is a pretty tight and snug fit so you don't have to worry about aligning this up, it's the LCD screen you have to worry about.
With all these factors, you have to navigate all of this and trying not to get any dust or finger prints on both the LCD and Digitizer, while your at it, it's recommended that you clean off the factory thermal paste and get some new arctic silver thermal paste, this will help with the cooling.
Trying not to scare you if your going to attempt this yourself but take it slow and don't rush in taking your Switch apart, to replace the LCD, you pretty much have to take everything apart, take photos if you have (or use google pics of tear downs) to know where everything goes when putting it back together.
This is talking from my experience, I'm confident in doing these kind of things but for someone who isn't, if your not careful you can pretty much stuff things up.
Good luck.