The Why does my printer say its OFF LINE??and Can't keep wireless c7250 all-in-one on line? threads are certainly ones that affects a lot of folks, but there are a number of causes and workarounds and may be dozens of "solutions" that apply for a particular combination of printer model, OS version (including SP levels), connection method (USB, wired LAN, wireless LAN) as well as computer and printer power saver settings. This is certainly not a one size fits all solution.
It is also interesting that in both these threads the post marked as the solution is not really the solution but rather the OP indicating that a previous post solved his particular issue.