Locked out of the admin interface I'm having an issue with the login-page and since this is my first Craft website, I'm a little unsure of where to begin to look. 

When attempting to log in to the admin interface, my username/password is rejected only with the message "An unknown error occured". The same goes for trying the "Forgot your password?" function.  

As far as I can tell, the only change recently made was running the auto-update to the latest version (2.X) last week. Have not logged back in since, so this could very well be the culprit. The front end loads just fine. I do have a couple of plugins installed, but not sure how I can deactivate them without using the admin interface. 

Are there some log files in the /craft folder where I can start to look? 

Any help much appreciated.
I don't have enough rep to comment, but I can answer a few of your questions.

Craft logs are located in /craft/storage/runtime/logs.

Make sure you have a backup of the database before you uninstall the plugins. Depending on the plugin you could experience data loss.
To remove the plugins you can go to the /craft/plugins directory and delete the folders. 

Keep in mind if you do this it will not allow the plugin to clean up after itself. 

I also found this which may help avoid the clean up issue. https://github.com/khalwat/uninstall. It is a tool that allows you to uninstall plugins via the cli.