I ignored a file in TortoiseSVN by mistake. How do I reverse this and add the file to my repository?
If you right click on the directory containing the file and select SVN Properties, you should see an svn:ignore property with a list of ignored files. Simply edit that list and remove the rule ignoring your file.
Just edit the svn:ignore property (in the subversion tab of the directory properties).
The help states
If you want to remove one or more items from the ignore list, right click on those items and select TortoiseSVN → Remove from Ignore List You can also access a folder's svn:ignore property directly. That allows you to specify more general patterns using filename globbing, described in the section below. Read Section 4.17, “Project Settings” for more information on setting properties directly. Please be aware that each ignore pattern has to be placed on a separate line. Separating them by spaces does not work.
See chapter 4.13 of the help for more details.
I usually just add the ignored file with Tortoise and don't bother to remove the entry from the ignore list. so far I had no negative side effect...