Trying to stage changes fails every time with a message that index.lock already exists. However, opening Explorer and navigating to the .git directory reveals that the lock file does NOT exist. Trying to stage a file with all other programs closed doesn't make a difference. I can open a terminal and use git commit without a problem. Here's the error:

git -c diff.mnemonicprefix=false -c core.quotepath=false reset -q – platform/common/src/org/jw/service/library/LibraryManager.java
fatal: Unable to create '[my project path]/.git/index.lock': File exists.

Another git process seems to be running in this repository, e.g.
an editor opened by 'git commit'. Please make sure all processes
are terminated then try again. If it still fails, a git process
may have crashed in this repository earlier:
remove the file manually to continue.