If you have iFile, download an older TaiG untether deb file and an older cydia installer deb file.
Once you install those two, Turn phone off and back on. Cydia should open. Then upgrade those two in the changes tab in cydia. Any problems still PM me because I had the same problem.

Everyone telling you to boot into safemood does not know the type of problem you have.
This problem happened to me today and what I told you fixed my phone.

Ones I used to install:

If you can't boot into no substrate mode to access Cydia then you might as well restore and jailbreak again as a simple option. 8.4 is jailbreakable so just do it again.