Ensure your device has sufficient battery life. A low battery might cause issues with the normal boot process.
The bootloader is triggered by holding Volume Down during startup. If your volume key is physically stuck or pressed by a tight protective case, the phone will boot into this menu every time. Try removing any case and clicking the volume buttons several times to ensure they aren't stuck.
Use a PC to run a "no-wipe autoloader" for your specific model (e.g., BBB100-1). This can often fix boot loops without erasing your data if the OS is merely glitched.
Stuck in Bootloader Mode - BlackBerry Priv - CrackBerry forums