Android on device testing doesn't work anymore, used to

TLDR: on device app testing doesn't work anymore, used to perfectly.

I'm currently writing an android app, not a super complicated one, just a small one with text to get the hang of it. Since I have an old AMD cpu, I can't use the emulator, so I test on my phone. Everything worked perfectly, I could see my main menu, until I added a second java class/activity: the app builds with success, gets transfered to my phone, I get either a blank screen or it prompts me to the usual security "are you sure you want to run it?" before giving my a blank screen. I don't understand, it worked perfectly and quite well, I could use the app, type text etc, but now I can't even run it... also, when I go back on my homescreen and try to tap on my app icon, which used to work, it just spashes a blank screen, again.

I tried uninstalling my app and reinstalled it, did not work. Restarted android studio, my machine and the phone, did not work. I removed dev mode on my phone before putting it back on, same, did not work.

IDK if it's important, but since I'm learning/in the building process, all the UI elements (e.g textviews, textedits, buttons etc) are laid out BUT not all of them are referenced in the java code.

Thanks a lot.