Hello! Thank you for your question. I will be assisting you today.
I'm sorry about the issue.
1. Please press & hold the power on/off button on the nexus for about 20 seconds, then turn it back on and check.
2. Go to Apps -> Settings -> Application manager (or something similar), then go to ALL apps (swipe from right to left as needed), scroll down, and select the item "Google Services Framework", then clear data and clear cache, then check. If needed, power off the nexus, then turn it back on and check.
3. If that still does not work, the software is corrupted, please connect the nexus to the computer, and back up anything you need, such as photos, then do a reset:
When you are available, please type at the box below, then click on the Reply button at the bottom in order to communicate with me. Thank you,
OK I have done that and it seems to have worked - so no interruptions so far. I notice at the bottom of the screen there is a 'Safe Mode' now. Many thanks. If the problem reoccurs I will try your other options.