Try starting it up in "Safe Mode" and see if it will fully turn on. If it does, your access to things will be limited, however then you know the problem is software. So virus, malware, bad app, etc., which you need to remove. If, however, it won't load even in "Safe Mode" it may require a "Hard Reset" of the device, which would wipe everything on it, or possibly even be a hardware problem.
In case any data loss in the process, you can recover the lost data by referring [https://howtorecover.me/minitool-mobile-android-recovery-review|the guide].