2004 Accord VTEC - immobilizer issues

Sorry that my first post is asking for help but am really stuck.
So the Accord was fine until a week ago and all of a sudden the car would not start and the green flashing key was there.
So I purchased the HDS HIM interface (the double board version) and it connected to the Accord just fine using HDS 3.105.012
However, as soon as I try and get into IMMOBI it asks me for a password and although I was given a range of different ones, none of them worked.

So looks like I might need to find a way to buy a password.

However, I am now thinking why did the key suddenly lose it's programming? Am I jumping to conclusions that the key that was working, needs reprogramming?

Can someone explain the immobilizer system for me please? Is it all controlled by the ECU and the antenna in the key slot or is there a separate immobilizer box. What could be the issue - is there something that happens frequently that I could check before diving into reprogramming? Just worried that I might get a password to get into IMMOBI and then find that there is actually another fault

Really appreciate any guidance - thanks!