I recently performed a factory data reset on my device and now the play store is not working. No app is being downloaded Play store is not working at all. The downloading of an app starts and never actually shows 1% and only the box shows null download.
I'll take a shot in the dark....

IF Google apps (including google settings, WhatsApp, youtube, etc.) in general, are not working/are missing (perhaps you are even trying to re-acquire these),

and/or IF settings > accounts and sync no longer shows your Google account &amp; you can't just add it.

THEN
    I may know what's going on (from personal experience, also in GB)....

Your problem may be that Google doesn't recognize your phone without the Google apps and *especially* without the Google account it expects to be on/associated with that phone. This despite that your device's ID #s haven't changed.  (pretty sure this is a safeguard against 1) spoofed device-id numbers, 2) the fact that the usual reason to factory-reset a phone is that the phone is going to be returned/sold to someone else)
Your solution (in only the case I have guessed) would be to acquire a "gaaps" package (yes, that's short for 'google apps') from a release date, platform appropriate to your device. (i.e. don't try to install the current lollipop apps on your GB device). This should include Google's sync and account settings background app and one-time-init (for initial set up upon purchase).
 If this sounds like your situation, just search for gapps or gaaps.zip.
 If this **doesn't** sound like your situation, just follow the advice above and provide more details. (did you just root/switch ROMs would also be relevant)
    good luck
    **-R**