It is not uncommon nowadays for adults to move back in with their parents. The reasons are many and varied: an unexpected loss of employment, marital separation or divorce, resumption of studying being among them.
There's a lot of personal issues to work through when moving back into "the nest," certainly. But you shouldn't forget that it's also important to make the new living arrangements as easy for everyone as possible. Here are a few ways to do this:
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Discuss payment before you move in. Rent is a very reasonable option. If you can't afford that, then you can try to take care of some of the utilities, groceries and other household necessities.
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Give yourself a deadline. You don't want to be living with your parents (again) forever. Chances are, they don't either (for your sake as much as theirs).
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Ask about house rules. You may be an adult, but it's their house. Also keep in mind that the rules may have changed since you first left. Talk with them directly about it. If there are other members living in the house (a younger sibling, for example), be sensitive as to how your presence may have affected their routines and boundaries.