Enter note and zoom-in suddenly stopped working

I am using Mac version 1.1 because I have an old version of Macbook. It worked fine until a few days ago, the functions of entering notes and zoom in stopped working. I have tried to create a new file but still to no avail. I don't know if it is a bug or I had hit a wrong key? I have tried to find the solution on the forum, but could not find it. Is there any way to restore the functions? Thank you, Fred