Do I need reglass or LCD replacement?

A black screen is almost always the result of a failed display, and the normal fix for that is to replace the screen. Probably something like over 95% of the time that will fix the problem.

As far as the actual replacement, there's a video hosted here on iFixit showing you how to do that replacement.

Samsung A71 (A715F) Original Screen Replacement - iFixit Repair Guide

If you prefer the written, step by step style of guide, there's one over on the Spanish site Nadie Me Llama Gallina. It's in Spanish, but used with the Chrome browser's translation, it's quite usable even if you don't speak Spanish.

Manuales / Samsung Galaxy A71 / Fullscreen

There's another one at the German site iDoc that has native translation to English built in if you prefer.

Samsung Galaxy A71 screen replacement - repair guide

Make sure the replacement screen you purchase specifies that it comes with the frame attached; that's the way these guides assume your screen will be, and is the recommended/official way to do it. It is possible to replace just the screen without the frame and you will save a couple of bucks, but that requires a completely different set of instructions and you have to glue it back on yourself. Buying the glue will probably cancel out the small price difference between the display alone and the display plus frame assembly.

Hopefully that's enough information to get you going. Good luck with your repair, and let us know how it goes!