Okay, I realize what went wrong.
One time, when I was trying to delete the wordpress folder from XAMPP htdocs, windows gave me a “cannot delete” pop-up. I saw some font of SI CAPTCHA was still in use from XAMPP’s “httpd.exe” process. So I used unlocker to break the process.
It was this action that caused the problem described above.
The solution is to restart the XAMPP control panel & restart apache/mysql. Then everything will work again.