Older laptops 3+ years or so, often burn out the monitor. Plugging in an external monitor will often help by giving you access to the laptop if the problem is the screen itself.
If you plug in the external monitor and the system comes up, then you have (most likely) a bad screen. If it does not come up, you may have more serious issues.
They used to use a florescent lamp(s) to light the screen. They can be replaced, but if you are not doing it yourself the labor is likely non-economic.