MacBook Pro A1278_dark screen

You can try the steps here and see if anything helps:

If your Mac starts up to a blank screen - Apple Support

If not, if you have access to an external monitor you can connect that and see if you can get that to work. It'll probably require an external keyboard and mouse so you can run the Mac with the screen closed to get it to use the external display as the primary monitor.

If that works, then my guess is that the screen, perhaps the backlight, has failed. If the external display doesn't work either, then I'd presume that the graphics circuitry has. Either way, a twelve-year-old Mac would in my opinion not be worth spending any money on.

Regards.

The graphics card may be failing and so be intermittent. The same could apply to the screen itself, or several other components.

As to connecting an external monitor, that's a way to try and isolate the problem since there's no way to swap internal components as could be done in a modular desktop-type system. If you don't find that suggestion worthwhile, that's your choice. You can try Apple's diagnostics:

Use Apple Diagnostics to test your Mac - Apple Support

but whether they'll work on such an old system and whether they'll turn up anything if they do, I can't say. But you can try it.

Regards.

That would be my guess, yes.