I am using pages. I can see inspector. However i can not anymore open inspector. It does not open anymore a window. THanks
One very simple situation that hides the Inspector pane is when there is a Help window open and it is set to "Stay on Top". Make sure that there is nothing on the page that the Inspector could hide under, and look carefully around the edges of your screen to see if the Inspector may be almost fallen off the edge and not catching your attention.
I think this could be a symptom of corrupt Pages preferences. Go to HD > Users > (your account) > Library > Preferences. The user's Library is hidden in Lion but it is easy to open. In Finder, hold down the Option key while clicking on the Go menu & your user Library will appear about halfway down the list.
Others have had problems with inspectors that were on second monitors that can also be "cured" with deleting the plist.