You possibly switched your pages from quirks mode to standards mode by adding a standards invoking DOCTYPE, or changed the css. In any case, this is incomplete and will be ignored (from kent-telephones.ismywebsite.co.uk/CSS/menu.css line #72):
It should be:
font:Arial, Helvetica, sans-serif;
You can use just font (which is a combo property, like border or margin, both of which have component properties that can be set separately), but then you have to specify at least the weight, often the size and always the family, in that order, space separated on one line (other font properties can also be included, if done so in their respective order*:
font-family:Arial, Helvetica, sans-serif;
Given your current setup, it's easiest to just use font-family because you already have defined those other things separately, which is also OK to do.
font: 700 16px Arial, Helvetica, sans-serif;
Your other font problems on this and/or other pages are probably similar, if not, let me know:
The browser cache may need to be cleared and/or the page refreshed to see changes.
or your favorite css reference for the proper order and syntax when using the combo font property.