A deleted System font is a possibility. However, deleting System fonts is difficult to do and is unlikely to be the cause of the OP's problem--unless he is doing something really stupid. It is more likely that the OP installed a font at conflicts with a System font--or has installed a utility that modified fonts.
Cleaning the font cache is a good idea. If that does not fix the problem, then systematically deactivate any user-installed fonts until the offending font is found. Fonts installed as a part of common application packages like MS Office are unlikely offenders. First, go for the cheap [or free] fonts downloaded from the Internet.