Sticky notes, uses Segoe Print font as a deafult font, so if you have deleted this font form your system, then it will use Microsoft Sans serif as it's default font.
Also if you have copied something from a internet page or something in there, it will use that font instead, I suggest that you press on the + button and try typing on a different sticky note as if you still have the Segoe Print font installed on your computer, it will use that.