Hi @LIWeifan,

It sounds like you may have collapsed your course menu, which is something that Canvas remembers even after reloading a page.  You'll want to click the "hamburger" icon in the upper-left area of the screen to show the course menu again.  This is talked about in the "Collapse Course Navigation Menu" section of the How do I use the Course Home Page as a student? - Instructure Community - 504 guide if you need more info.

I hope this helps!

-Chris