On Linux, Unix, Git Bash, or Cygwin, try:
rm -f .git/index.lock
On Windows Command Prompt, try:
del .git\index.lock
For Windows:
From a PowerShell console opened as administrator, try
rm -Force ./.git/index.lock
If that does not work, you must kill all git.exe processes
taskkill /F /IM git.exe
SUCCESS: The process "git.exe" with PID 20448 has been terminated.
SUCCESS: The process "git.exe" with PID 11312 has been terminated.
SUCCESS: The process "git.exe" with PID 23868 has been terminated.
SUCCESS: The process "git.exe" with PID 27496 has been terminated.
SUCCESS: The process "git.exe" with PID 33480 has been terminated.
SUCCESS: The process "git.exe" with PID 28036 has been terminated. \
rm -Force ./.git/index.lock