Try going to settings tap on date+time and turn off Set date and time automatically and see if that helps. Make sure date and time are correct.
Also see how many apps you have running in the background. Disable a few and see if that also fixes the issue.
Are you a Microsoft Account with the phone? If so is that account accessible via your desktop browser; like going to onedrive.com and logging in?
If that doesn't work you might have to do a factory reset.