If macjack's solutions do not correct the problem, you will need to seek professional help. Such symptoms indicate failed graphics hardware, a failed display, of failed display cabling.
You can do a bit of diagnosis to see which one if you have an external monitor and a way to connect it:
1) If the problem also shows on the external monitor, the internal video hardware has failed and only a new logic board can fix it. Considering a logic board replacement can cost 1/2 to 2/3 of the cost of some new Mac notebooks, you have to decide to repair of replace.
2) If the image on the external monitor is unaffected, that point fo the dispaly itself or the cabling. A disply is not cheap but probably less than a logic board for a non-Retina MacBook Pro. Cabling could be the simplest fix.
NOTE: The age of your MBP bring into play Apple "supported/vintage/obsolete" repair policy. In most places, Apple does not provide either parts or repair service for computers that have been out of production for more than five years. Your five years on that nmodel ran out last June. For the full policy, please see this:
Vintage and obsolete products - Apple Support
Although Apple will not fix it, some Apple Authorized Service Providers (AASP) may. It depends on if they have parts inventoried.
