In ground pool liner pulled away from wall about 8 inches how to get it back?

If possible, lower the water level (this is not always an option for really big pools). Some people recommend using a hair dryer to warm of the section of liner around the lip that needs to be re-inserted. For obvious safety reasons (hair dryers and pools do not really go together), I have used a couple pots of boiling hot water poured over the area to soften up the liner, and then used brute strength to force the lip of the liner back into the groove. The lower the water level, the easier this is (because you're pulling against less weight). If the liner keeps popping out, you can wedge in a popsicle stick (or something of equivalent width) over the top of the re-inserted portion. Pool stores also sell liner "locks" that look a little more professional and are likely to wear better.

Try using very hot water on the liner.