My top half of the screen isnt working.

When your touchscreen fails, there is really nothing that can be done to fix it and the only solution is to replace it. Of course, virtually all modern phones have the digitizer fused to the LCD display, so you'll be replacing the whole screen assembly. A quick check online shows that replacement screens are readily available and inexpensive; the first one I noted was only $20 USD which is cheap compared to newer OLED displays.

Your phone is similar to many Android phones in that there are two ways to replace the screen. The recommended way is to purchase a display that's already mounted to the midframe. That way the LCD is already fixed to the frame so you don't have to worry about gluing it on and making it water resistant. Doing it this way means you have to remove all of the internal components from your original phone and transferring them to your new screen+frame assembly. It's a fair amount of work, but the phone is designed to make that process fairly simple. Of the written guides I was able to locate (unfortunately iFixit doesn't have one), this is the method they use. Here's a set of step by step instructions from the Spanish website Nadie Me Llama Gallina.

Manuales / Samsung Galaxy M20 / Fullscreen - Nadie Me Llama Gallina

The site is, of course, in Spanish but offers some amount of translation to English. I suggest using it with the Chrome browser and its translation feature, which makes it work well no matter what your native language.

The other way to replace your screen is to separate the LCD and digitizer assembly from the midframe and just replace that. The screen alone is generally cheaper than the screen+frame assembly, but only by a few dollars. If it doesn't come with glue, you'll make up the price difference having to purchase adhesive that will secure the screen back on. If you were wanting to reuse the screen for some reason, this is not a good way to do it, as it's easy to damage the original doing it this way as you'll see in the linked video where they ruin the old display getting it off. Once you have the old screen off, then you have to clean all the old adhesive off the frame, spread glue around the perimeter, then clamp the new display onto the frame until the glue dries. Here's a video showing you how to do that, but again I would recommend doing the screen+frame method instead; however the choice is up to you.

Samsung Galaxy M20 Screen Replacement Complete Tutorial - YouTube

Whichever method you choose, you'll be putting on a new digitizer that will restore your touchscreen functionality and fix the issue you're having. Good luck and let us know how it all works out.