Whenever I start my MacBook (mid 2009, running OS X El Capitan 10.11.6), like any normal day, I click on my main account and log in. 

Today, when I tried typing in my password, it let me in as usual but I get nothing but a black screen with the spinning beachball cursor. However, I can log in to my other account perfectly fine. 

I've tried booting into safe mode and logging in but it just gives me the same result. I've also run First Aid on the disk, but that didn't resolve the issue either.

How can I troubleshoot this further?