Yes, many of us have had the same issue. Here is the very strange solution. But it works! And it is easy to do.
resetting the SMC should enable you to boot once more (but the problem will come back next time you shut down) . this is how
https://support.apple.com/en-us/HT201295
Once you have booted:
1) Open Terminal
2) Enter "system_profiler SPExtensionsDataType > ~/Desktop/kextList.txt" without the quotes and hit return(this will take a while to run).
3) There should now be a kextList.txt file on your desktop, open it and press both the "Apple(Command)" and "F" keys to bring up the find.
4) In the find field insert "Obtained from: Not Signed" Copy the destination to the .kext file to a list for use later. (Click next to cycle through all of them.) Example: /System/Library/Extensions/JMicronATA.kext
5) Browse your drive to /System/Library/Extensions and remove any of the unsigned kext files.
6) Reboot and you should be all set. one last SMC reset might be needed.
The problem is that el capitan 's installer does not properly clean up your extensions folder of pesky .kext files left over from applications you no longer have and some are incompatible with el capitan's boot up process. You have to do it yourself.
Good luck. This should solve your problem!
if the sms reset doe snot help you boot, get another mac for a few hours: start up yours in target mode (press T as you start up) with the two computers connected to each other via firewire or thunderbolt and make a clone of your hard disk onto an external hard disk with carbon copy cloner . then restart your mac via internet recovery (command option R) and reinstall your original system, then download el capitan again and reinstall it. then use the clone you mad of your hard disc to ,migrate all your settings and data and apps using Migration assistant. Then you still have to do the stuff described above
it takes a while but you will have lost nothing.
best