Linea Recta formulated on Thursday :
My default browser Chrome got changed to Explorer.
Is it possible this is done by Windows update?
Did you install any other software? There are a lot out there that
will put unnecessary garbage on your system if you don't pay attention
during the install, including changing your default browser, home page,
another toolbar, etc.
You didn't give too much information to go on, but this is one source
event that can change your default.