It could be a number of issues causing this.
Try powering off the device. Hold the Volume Down button and plug it in to the wall charger and keep holding the Volume Down button until you see a screen with an exclamation point then quickly release the button.
Now hold the Volume Down + Power buttons until the phone vibrates and reboots.
If you still have the wait time to unlock you will have to reset your phone. Use the second tool on this page:
How can I recover/reset/restore my phone software? - Microsoft - USA