Ok - first thing - I have fixed the problem - however I was more than slightly worried so I thought I had better ask why it happened.
First of all I have the latest version of 1Password which is downloaded from the website (not app store).
Today I upgraded from Yosemite to El Capitan 10,11,1 and everything worked fine. So maybe I was getting a little too adventurous but I thought I would change my 1Password password. I have about 9 vaults so I chose a less vital one and went through the option to change the password. All appeared to work ok. Before I did this I made a copy of the 1Password folder by copying it from dropbox into my downloads folder.
A while later (and I'm not sure if there had been a reboot or not) I tried to load the desktop part of 1Password and it just froze. I forced it to quit and repeated the process with no joy. I did a reboot and still no joy. It did not appear on screen or prompt for a password - just froze.
I did another reboot and tried again. At this point I recall the main program popped up a message saying that it could no connect to the 1Password Mini.
I did a full uninstall using the uninstructions from your website, rebooted, deleted all the trash, made sure there were no other versions on my machine and then did a reinstall again. Once more - no joy.
At this point I wondered if the password change had screwed up the files so I restored those. I also moved the backup of the files onto a network drive and removed them from my local machine in case the app was confused by which version to load.
Another reboot and another install and it finally works. I don't know what broke it and frankly I don't know what fixed it.
Any insight would be helpful. El Capitan - Password change - something else ?
1Password Version: 1Password 5 Version 5.4.1 (541003) Agile Web Store
Extension Version: 4.4.4
OS Version: 10.11.1
Sync Type: Dropbox