There are an increasing number of posts in this forum and elsewhere in the internet with the steps to do a resets on different printers. The reset itself will sometimes fix problems, so the reset is not the issue.
Some resets require you to enter button combinations to open special menus on your printer that you were never meant to access. These special menus are used by the developers and support personnel for debugging issues, but they are not meant for untrained users and can have unintended results. One wrong button push and you no could no longer have a printer, but instead a pricey paper weight. To top it off, if you do happen to "brick" the printer this way, it is considered customer induced damage, and voids the warranty automatically. These resets are only meant to be given out and used as a last resort, and under direct instruction of a HP tech support representative. They should not be done lightly!
If the printer has already been damaged due to fiddling in the support menu's you may need to Contact HP for (paid) service. Best to not do the damage in the first place...
An example of the damage that can be caused would be printers that use special setup cartridges. Resetting the printer to factory conditions may cause it to once again require the setup cartrdiges. If the original setup cartridges have been depleted or discarded the user would need to contact HP for setup cartridges since these are not available from retail channels.