A lot more information is necessary in order to help you. Did you update godot version? Are you using stable or older alpha? Did you add anything that may be causing issues in between? Did you accidentally change your export settings? Did you phone update? Is there a new dependency issue? Is there an error from the app using log cat? Are you building an apk, or you are using the build through USB button in the editor? IF you are using the build button, is it building properly and failing to launch, or is it failing during build?
I sure hope you can get this problem sorted out! But as you can see, we can't really help unless we know more, anything we say is kind of a hit in the dark. Do you mind being more specific?
That being said, android export can be touchy at best. Some things I would try are removing the splash screen (for some reason that sometimes causes issues), checking your open gl supported version (it could be that you added a 3d feature that is not supported by your device maybe?), and resting the export settings to default.