I tried to attach it to an external monitor and got nothing. I don't understand how it could work one minute and not the next. How do I know if it a motherboard issue?
If you can't see anything on the screen you are pretty limited in what you can troubleshoot.
If you're sure nothing is on the monitor after toggling the screen outputs then the problem appears pretty major and not easily fixed.
As for how it could happen? Well things fail. You had a warning of some sort and now it failed so it sounds pretty certain it's a hardware failure that is not fixable by an average person.
So, unless the laptop has a Warranty or is expensive enough to justify repairing I'd guess it's toast.
If so, I recommend removing the hard drive so you can recover the data either by using a 2.5" USB device like BlacX to slide it into, or just attach it to a desktop PC via the power and data connectors.