A random reboot is perfectly normal, I get them from time to time, it should not affecting the JB process after the reboot has accrued.
Basically people experience their Pangu app crashing due to the 1 year certificate expiring. But we don't need this app anymore!
Going to
http://jbme.qwertyoruiop.com
and hitting "go" is all you have to do, although it may take a few tries, you will eventually end up jailbroken again.
If you did not install the "Patch" from Cydia then you will not be able to run
http://jbme.qwertyoruiop.com
You will in fact need to run the Pangu tool via a computer, simply get the latest Pangu IPA from here :
https://en.pangu.io/
Sideload the IPA with Cydia Impactor
Run the Pangu app once.
When you have Re activated go to Cydia and get the jbmepatch, this will allow you to then use :
http://jbme.qwertyoruiop.com
to reactivate in the future.