That is a good question Mike-do you have a backup?
I just updated my blog site/s to WP3.5 & my main site has crashed giving me an error message that there is no Theme directory (but the Theme directory is present on my server). Why can I still access some of my other sites?
For me, I expect this crash will have been caused somehow by updating to latest BuddyPress plugin version which gave me a few lines of php error code. I tried deleting the plugin & renaming my plugins folder which has worked in the past but not now. I spoke to my server live chat and the op said they could only do a WordPress restore for a fee-great! He also said lots of WordPress users had problems with this update?
I will try & do my own fix but this is a right royal pain...
Unfortunately Mike I do not think there is a roll back option in WordPress. You might try disabling/deactivating/deleting the offending plugin & or renaming your plugins folder entirely. It may also be a good time to check for theme & database backups & create them if necessary as a restore/reinstall of WordPress maybe necessary.
Goodluck with fixing your install & I hope I can get my site fixed too!