Sorry to hear there's trouble with your display. There could be a number of reasons for it to have gone out. None of them are easy fixes.
The simplest possibility is if the display connector has become loose or corroded. You must open the back of the MBPro and carefully unplug the connector from the logic board. It's not a simple "pull it out" however. And if there is corrosion it may not be possible to clean it sufficiently to fix.
Second is if the internal display cable has gone bad (from flexing over the years). While it can be replaced, it requires a very involved, delicate disassembly of your laptop. It's definitely not something for the average DIY'er to do.
Other possibilities may be that either the backlight or the display may have gone bad. That would require the complete replacement of the display unit.
Bottom line is this ... for a 9-year old laptop it's probably not worth the cost to fix. You might get along by using it with an external display, but then you lose portability ... and it's still a 9-year-old laptop with a major problem. Time to consider replacing it.