So the title sums it all up pretty well. I'm making mods specifically for 1.12.2, if that helps.
Any time I export a mod into a .jar form, it doesn't work when I put it into the mods folder. Let me expand on that: I fire up the game in forge, with the mod in the mods folder and.... Nothing.
Now, this isn't something that happens every time. With a previous version of the mod that I no longer have, I got the game to fire up, but it wouldn't even make it to the Mojang logo. All I was presented with would be a black screen where nothing happens. If I click the window, it wouldn't respond and eventually crash, leaving no crash report.
Now with the modern version I've made and can present, I can put it in the mods folder, but when I fire up the game, it acts like it's going to start, but no window opens, though the process does run in the background.
But are you able to test the mod inside MCreator? Does the test Minecraft client environment start?