I believe pangu8 is one of the scam sites. You should be able to rejailbreak if Cydia is there already by going to http://jbme.qwertyoruiop.com
You can try sideloading the official pangu app if you have computer. Download it from: http://en.pangu.io
Then install .ipa using Cydia Impactor.
If you don't have a computer, you can install the jailbreak app right from your device here: http://j.25pp.com/ota_tutorial
It'll ask to open in app store, tap yes. If it fails to download/install, hit retry and turn date back to May 15 2015. It'll then install. Reset date to current date, go to settings general profiles and device management and approve profile for that app (hit "trust" or "verify"). Then turn date back to May 15 2015, open jailbreak app, uncheck the box under the big circle, hit the big circle to begin jailbreak, then lock device a couple seconds later and wait for respring into jailbroken state.
Either method may take a few (or several) tries. You'll know it worked when you see error message saying "memory almost full" b4 respring.
Edit: If you still can't rejailbreak, check out these links.
http://www.redmondpie.com/fix-re-jailbreak-with-pangu-pp-app-not-working-after-reboot-heres-how/