loss of sleep mode

[Bield]

Since upgrading to android 5.1 - when pressing for sleep mode the tablet starts to reboot to the opening page and then goes through a continuous cycle of going to sleep and then rebooting. The only way to stop this s to power off. I have tried a factory re-set. Another feature is that I can no longer get to safe mode with the power off button. Effectively, I have lost the sleep mode

 

[B. Diddy]

Welcome to Android Central! By "pressing for sleep mode," do pressing the Power button to turn the screen off? Is the tablet rooted?

Try reinstalling the firmware manually: How to manually update your Nexus | Android Central