My paint can lid was left open and the paint is dried up. How can I restore it so that I can use it to touch up my wall?
Nothing that I'm aware of can restore dried paint to a workable liquid. What you can do is take the original can to the store where it was originally purchased and buy some more, using the original can for reference. Most likely this new paint won't be a perfect match but it might work.
In the future, small amounts of paint can be put into a jar or other well sealing vessel for future use. Try to always keep the original can for reference.