Cracked Macbook Pro screen can't get external monitor after booting Sierra.

I have a 2013 Macbook pro Retina with a completely unusable cracked screen. I was using an external monitor with El Capitan but when I updated to Sierra I can't get an external monitor to work and don't know what to do since I can't see what to put in next. I tried my Apple login and that didn't work. I tried Command F1 and that didn't work. Any help would be great!