After upgrade your Windows Phone to Windows 10 build 10136, you may see the issue related to A1B2C3. Here is the detail from an Insiders:
“I installed Windows 10 Insider Preview Build 10136 on my phone. Now my lock screen is asking me to type “A1B2C3”. I typed this in and was told that I had one last try. So I pressed “OK”. Then the lock screen appeared with on the wallpaper and no PIN pad. If I slide up, it tells me to type “A1B2C3” again.”
“This is a known issue in Windows 10 Insider Preview Build 10136 for phones.
If you have too many PIN unlock attempts, you’ll see the “enter A1B2C3” reset experience. However, there is an issue in this build where after you enter the code you won’t see the PIN pad. The workaround is to press Emergency Call after entering the code, then press Back and you’ll be able to enter your PIN.
This phone build also enables a new “double-tap-to-wake” feature. If you’re seeing the “Enter A1B2C3” experience frequently, you may be accidentally double-tapping the touch screen while the phone is in your pocket.
We recommend disabling the double-tap-to-wake feature on some Lumia devices by going to the Settings app, and then Extras > Touch > Wake to prevent any accidental PIN unlock attempts.”
Hope this information helpful for you. If you have any issue more, please leave a comment below, we Insiders will try our best to help you.