system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes someone to forget something they use basically on a daily basis, such as a phone number, debit card pin number, etc?

I use a gas card for work, and after using the same card with the same pin for 3 years, I completely forgot what it was. Could not for the life of me remember it.
gpt
Think of that information as a destination. That memory has a physical location in your brain. To get there, you must follow a neural path. For whatever reason, that path is blocked one day. The memory still exists, you just have to find another way to get to it. 

This is where mnemonics are helpful. It's like a detour on that road. You have to go a further distance (like having to remember the mnemonic device on top of the info) on the detour, but it still leads to the same place. The more paths you have to the destination, the better off you are. 


Neural paths are actually a lot like real paths, the more you use it, the easier/faster it is to traverse it. 

The hard part is figuring out why that path was erased for you. It could be that you learned something similar and the paths physically interfered with each other.