Look in the .svn/entries file for the URL of the repo.
Get the number of the last revision the file was in before you deleted (or if you don’t know, start from the current one and work backwards)
svn cp -r <rev> <repourl>/<filename> ./<filename>
Then commit.
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And for the special case where you accidentally commit the same file with two different case names on windows, which the windows svn client chokes on, the solution is to delete both files, by deleting the one case of the file, updating, then deleting the other case of the file. Then if you want to get your file back, logically one would use svn cp -r as Rachel suggested, but this doesn’t work, so instead you can update to the previous revision when the file did exist, filesystem copy it to a new name, commit the new named file, and then do an svn update.