Macbook Pro black screen during logon
I had exactly the same thing happen. Upgrading to El Capitan resulted in my having to erase the entire hard drive and restore using my back up! Thankfully I always back up but so far I'm not happy with ElCapitan. This upgrade has brought so many problems which are ongoing. Apple are very helpful and can offer you a number of 'fixes' so I'd recommend contacting them. Sadly for me as I mentioned erasing he hard drive was my only solution. Good luck