According to reports, some Nokia Lumia users are experiencing issues with the power button. To be specific, the Lumia handsets affected are the Nokia Lumia Icon handsets where the power button is being unresponsive. Some have reported that it now takes about 2-3 presses of the power button in order to wake up the handset.
Assuming that this is affecting your handset, it looks like you will have to reboot your handset. For Lumia Icon users, this involves holding down the volume down and power button together for about 15 seconds, after which the phone should reboot itself. Hopefully this is an issue that Microsoft has been notified about because the last thing Microsoft would want is for the update to break any devices.
This is currently the second problem that we’re hearing about the Windows Phone 8.1 update. Previously there was a report about how Cortana could not set the alarm on Windows Phone devices, instead issuing an error message. However we should note that this is the developer preview and that the final version has yet to make its way to end-users, so hopefully Microsoft will deal with all these bugs and issues before then.