When i try to put my PIN in there is no OK button, so I can't use phone

The same happens to me. Any help appreciated.

Same here after a software update. I found out that pressing an OK button is no longer necessary, you just need enter the correct pin and wait a second. Unfortunately, the software update has changed the pin! 

Tried rebooting, removing SIM, clearing cache partition, no luck. Only fix was to factory reset the phone, which means losing all the data. Even then, during the setup process it still asked for the previous pin (this time with an OK button, and from which we confirmed the old pin simply wasnt working) but fortunately it gives you another option to login with your Google account and password.