This is normal. You have probably returned to the category you were in and can’t see ones ‘further up the tree’. Click the underlined home button at the top of the page and you’ll always get back to the start. There is also a little folder at the top left of the box you are in when this happens. This will also show all the other forum categories.
Hovering the mouse over the icons and buttons will show text hints of what their function is.