What to do when ZTE Sonata 3 won't wokr after accidentally turning on DFNDR app?

I have a cricket ZTE Sonata 3 I accidentally turn on the DFNDR app and now I cant get my phone even back to a screen apps to turn it off and my phone wont let me do anything how do I fix this please someone help me out im not very familular with this phone yet haven't had it very long and am technically challenged as it it I hate these these touch screen phones not good with computers

Can you go to Settings > Apps > All > Dfndr and force stop it?

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