website stopped scrolling - html mouse won't scroll chrome firefox

Usually when I go to a website and use my mouse scroll wheel - I can easily scroll down the page without having to use the scroll bar.

On a website I built a couple of years ago - the scroll stopped working for some reason.

After a little bit of troubleshooting - I realized I needed some additional code.

I threw this in and walah - it works. I can scroll the website again with my mouse.

html, body
{
 overflow-y: auto;
 height: 100%;
}