Yes I had the same Problem with Lumia 520 and Windows Phone 8. The start screen did not appear. Only "Loading ..." was visble.
Whatsapp caused it when I tried to send a photo.
A Switch off and on of the phone or a hard reset with Settings>Info>ResetPhone had no success.
But the Windows Device Recovery Tool helped. See here: https://support.microsoft.com/en-us/help/12379/windows-10-mobile-device-recovery-tool-faq.
Download it to your PC and connect the phone. It is described on that web site in "How do I use the tool?" See also "Do I Need to do anything before I use the tool?"
I was afraid I would have to buy a new phone with Android OS ... ;-)