This is a simple and specific example for when someone has deleted a user in Umbraco 7 and now the pages aren't loading in the backoffice.
One thing you can do is to right click on the node and try to click on the rollback option. If you have custom errors turned off you should see an error message saying something like No user found with id N.
So now we know we are dealing with this type of issue we just need to run some SQL to fix it.
In these examples I am replacing the user id value from the old id of the person who was deleted to the new id of the super user, user 0. You can replace it with a different user id if you want.
UPDATE umbracoNode SET nodeUser = 0 WHERE nodeUser = 5 UPDATE [dbo].[cmsContentXml] SET [xml] = CAST(replace(CAST(xml as NVarchar(MAX)), 'creatorID="5"', 'creatorID="0"') as ntext) WHERE xml LIKE '%creatorID="5"%' UPDATE umbracoLog SET userId = 0 WHERE userId = 5
That should be it. You should be able to load the pages is Umbraco now.
I hope that helps someone, even if it is just future me in 6 months from now.