Two possibilities - OK, number 1 - try the usual power off, power on - 'fixes most problems' solution, but I guess you've done that.
Next one - some apps can 'turn off' the auto turn off. GoodReader is one, for example, that turns off auto off during a WiFi transfer. But that turn off is enabled even if you don't commence the transfer. So it would be worth thinking of apps that might be running that could turn off this function and that might be running in the background.
And, of course, I'm assuming you've not accidentally set 'Autolock' to 'Never' in the 'Settings' app....