Your Git Bash installation (which uses Cygwin internally) got a bit messed up, perhaps after an update or if some files got corrupted.
Close all Git Bash windows.
Uninstall Git for Windows from your computer. Go to "Add or remove programs" or "Programs and Features" in your Windows settings and find "Git," then uninstall it.
Download the latest version of Git for Windows from its official website.
Install it again. Just run the installer and pretty much accept the default options.