It probably began after adding some fonts, or doing something to system fonts.
Did you touch (remove or delete) any of the system fonts? Those should all be left alone so if any of those were touched you may need the reinstall.
An easy thing would be to use Suitcase (or whatever font management you use) and disable ALL fonts you have added. Log out of the system, log back in. If it now shows correctly, then one or some of those user specific fonts were the problem (and not system fonts missing etc). Enable 50 % of them, log out and back in, try it again.. again enable or disable 50 % of the remaining or added until you can narrow down where and which ones were the bad ones.