No indication of what browser you're using, but if it happens to be Internet Explorer, click "Tools" on the upper toolbar, then select "Internet Options." At the bottom of the screen that opens, there should be a button labelled "Fonts." You can select which fonts are used for web pages where no font is specified. A "fancy layout" page can still override your selection to display what the page designer wants you to see.

Most of the fonts available on your machine should be available for the "web page fonts" selection, and most of the ones offered here will be proportional, although there usually are a couple of monospaced ones. If you do much cross-cultural browsing, it may help to select something like Times New Roman, Arial, or Verdana that are likely to have extended font tables on your machine so that you'll see all the accents & umlauts & such correctly.

Selections in the "text" box are generally more limited, and most are usually monospaced.

If you're using another browser, I can't be specific, but you'll likely find a setup somewhere in Tools, View, etc that lets you choose a default.

If it's not a simple change in your selected default font, it's possible the one that's selected may have been deleted or damaged, and your browser is defaulting to the "simplest availabe."

John