System Restore do not destroy your saved documents or recieved e-mail. And proccess is absolutly reversible.
But if you was infected by a virus you have a 99.9% of chance to have infected/deleted your current restore points.
What is the point of System Restore?.
System Restore helps you restore your computer's system files to an
earlier point in time. It's a way to undo system changes to your
computer without affecting your personal files, such as e‑mail,
documents, or photos.
Sometimes, the installation of a program or a driver can cause an
unexpected change to your computer or cause Windows to behave
unpredictably. Usually, uninstalling the program or driver corrects
the problem. If uninstalling doesn't fix the problem, you can try
restoring your computer's system to an earlier date when everything
Reference (is about windows 7 but the feature transcend since Windows XP until current windows 10.)