My pesto turned solid. Why?
Most pestos use olive oil, which solidifies at around 6 degrees celsius - just over the temperature of the average fridge. The shop-bought pestos you buy probably adulterate the olive oil with other kinds of oils.

In any case, you'll be able to eat your pesto just by leaving it out at room temperature for a few hours, or by just scooping it out and adding to your hot pasta.