If you are unable to edit the CP, ensure you are not in Read Only mode. The easiest method to check is to see if the URL link has /1 added at the end.

If it does not then you are in Read only mode, which can be rectified by adding '/1' to the end of the URL and searching:

If you see an Error message saying the CP is currently being edited by yourself:

Check to ensure no other tabs or windows are open with the same CP. Once closed, the remaining page with the open CP will become editable.