Look to ToolsOptionsEdit tab and uncheck the "Fixed decimal" check
box. (It's probably on and there is the number 12 in the input box.
Dropping it to 0 may be a good idea as a precaution for the future.)

This is the setting that people use when they want to enter cents, for
example. Set the fixed decimal to 2, and when they enter 1234, it
appears as 12.34 in the cell.