^ You were warned a gazillion times that installing the preview could brick your phone, or that you could lose all your data. That's part of the insiders preview deal.
If you made a backup using 8.1 before installing the preview, which is standard procedure, you'll be able to restore that after reverting back to 8.1. If not, then everything will be lost except that which you can copy from your phone to your pc.
On the bright side, mistakes are a good learning opportunity. ��
Good luck.