I've recently purchased this game after it being recommended several times and I'm really enjoying it. However, every so often, I can't rotate my characters anymore. Using the mouse, I can move forward and backward. Using the keyboard, I can strafe right and left. But I can no longer turn. Am I clicking on something that can be unclicked? I've read the manual and searched the internet, but can't find anyone else who has had this problem.

Personally I always change the default keyboard config to WASD-style (if possible).

In W&W I use the following setup:

W - forward
S - backward
A- turn left
D - turn right
Q - strafe left
E - strafe right

The rest is a matter of taste. I use R and F for looking up and down, Z as skip turn, C as center view.

In default setup IIRC turning is NUM4 and NUM6 for turning left and right respectively.

I have the same problem, and to expand on the OP:
1. it happens even if the default keys are changed. Checking the settings shows that the keys are still set and changing them to something else doesn't fix anything.
2. The issue persists across Saves and Reloads, even after quitting and restarting the game.

Everything else works, but unfortunately, the game is unplayable without being able to rotate the characters.

I have the same issue, can move but can't turn using either keyboard or mouse.
Issue persists across reboots.
The only workaround I've found depends on something targetable being in your field of view. If you can get the game to turn your party to face your target it will unlock turning again. Almost any target will work if there are no enemies, a cockroach or rat in a dungeon, a bird or butterfly when outside.