Macbook Pro will not log in
When I restart, I get to the main screen where I log in, I can then log in, but when it changes to the progress bar, nothing progresses and it just sits forever. I have tried booting into recovery mode and using first aid. I have also reset PRAM, but nothing helps. I also can't boot into safe mode. Holding shift just gets me to the normal startup screen with my user account. It does accept the password, but then nothing.
I am on El Capitan and my Macbook Pro is about 2 years old. It has an SSD drive. This happened before about a month ago and I had to reinstall everything. That worked, but it has now happened again. Any idea why this is happening and how to fix it?
iMac, OS X El Capitan (10.11), 3.4 GHz, Fusion drive