Few minutes back I took some screen shots. But they disappeared even though I did not delete them. And even I didn't get any messages that "can't take screen shots"etc. Why it happened like that?
Welcome to Android Central! Which phone? If you have the Google Photos app installed, open it, tap Menu>Device Folders, and look in Screenshots. If you don't, then use your stock Gallery app or your file manager app and look in the /Pictures/Screenshots directory (or /DCIM/Screenshots if it's a Samsung phone).