This is really two questions in one. As far as the first part is concerned you'll probably find the answer in the manual under something like Power Save. This can be turned on / off by selecting the correct button. It is possible that you accidentally hit this button when doing something else. It would help if you could post the make /model of your machine.
As far as the second part of your question is concerned - sorry, no experience! but again, post the make and model and it's likely that someone will be able to help.