I think I may have set something up wrong, but I can no longer log into my master site. The master site is now a blank page, and the admin login won't accept my passwords. The master database is still intact, so I think I set up a wrong site folder.

Can I access JMS info in the mysql database? Can I change/see the info I've set my sites to?

First, JMS does not save any information in the DB.
All the information are saved in the /multisites directory.

If you have defined a slave site that with the domain name of your master, perhaps you have converted your master into a slave (if you have the wrong DB connection).

I don't think this is the reason of the "blank page".
In general a blank page result of a PHP syntax error somewhere.

This is perhaps related to a permission issue.

The only know solution to identified such potential PHP syntax error is to add debuging traces into joomla.

I figured it out. In getting ready to upgrade to the newest JMS, I uninstalled the patches (which was what I thought the suggestion was), this reverted the configuration.php file in my master site to use the db prefix jos_ instead of master_

solved.