The file in question is likely .git/index.lock and it should be safe to just remove it if you have no other git processes running. Make sure a git-svn command isn’t hanging.
PS My usual approach to fixing git-svn problems is to make a fresh pull of the repository. Time consuming, but you can do it in parallel with trying to fix the problem. Have a little race between you and git. Of course, this only works if you didn’t have unpushed commits.