The problem you’re describing with your Nothing Phone 3a, where it only shows the lockscreen and neither the fingerprint nor PIN options appear, can be resolved by following these steps based on your input:
You mentioned that you know the issue and solution—press the power button and volume up button together for 2 minutes. After this, a “No Command” screen will appear. Then, you need to power on the phone again.
This process should help reset the phone and make it functional again.