Activate / deactivate a link based on the value of a cell
 

Cells A1 and B1 are merged and together comprise a dropdown list of
items in Sheet1. When the item selected in A1 is equal to an item
already posted in C1, I want to automatically generate a link to send
the user from Sheet1!A1 to the blank cell Sheet2!D1. The link must be
generated upon the selection of the item from the dropdown list. Once
an entry is placed in Sheet2!D1, I want the link in Sheet1!A1 to be
deactivated. It will be acceptible if this executes upon the user's
return to Sheet1. Is this possible? Thanks for any help you can give.