Macbook Pro keeps returning to the login screen
My laptop rebooted itself yesterday afternoon. Since then, I can't get logged in again. When I restart, the login prompt comes up almost immediately and looks like it's going through the normal login process. Then an error message is displayed (attached) that says "Your computer restarted because of a problem. Press a key or wait a few seconds to continue starting up." Regardless of what I do, the screen returns to the login.
The machine was purchased in 2011 and I believe it's running OS X 10.11.x El Capitan.
MacBook Pro 17", macOS 10.13