Possibly a bit more information for the real expert tech minded people - like where exactly you were when it started etc might be helpful.
Try verify the installation - you can do this in either Gog or Steam. In steam the option is under Installed Files and click Verify File Integrity.
I've only had this happen once and it was because the game was stuck thinking I was in dialogue with someone, I just had to find them and chat to them again but this was ages and several patches back so probably not much help.
