Look in the goddamn hole where the doorknob was like people have been
telling you to. You will see the latch mechanism. It is child's play
to figure out how it works, then use a suitable flat-bladed
screwdriver to either twist or pull whatever makes the latch release.

Yeah, I was about to resort to profanity myself. If this guy is still
stuck, he shouldn't be allowed to own a screwdriver.

Yup, but I'm sure some bleeding heart will come back and admonish me
for being "mean."