Can't see my bookmarks.

[flobadob]

I've noticed that after a while when I'm on the internet when I click my bookmarks at the top of the screen I can see an outline of where they should be but I cannot see them and it is clear, ie I can see the webpage which I'm viewing behind where the bookmarks options should be. This happened last night and I turned the computer off. Today it's happened again after being on for an hour. Does anyone know how to fix this?

[flobadob]

Can anyone shed light?