Is the cell locked? In the cell options, there is a checkbox that says "locked". This is not suppose to have any effect though until the sheet is protected.Right click, "Format Cells", "Protection" tab (last one to the right), two checkbox options and the top one is "locked" You might try protecting and un protecting the sheet. I have something go haywire on me once and that seemed to help.
EDIT: (using excel 2007) Ok, that drop down your seeing was possibly created by the Data Validation option for a cell.
Try selecting the cell, then click the "Data Validation" button on the ribbon (excel 2007). All the options for the drop down should be in there. The source and what is allowed is listed in the first tab "settings", the second tab is the "input message" (this is what comes up when cell is selected), and an error alert.
You can see the drop down options from the source by right clicking on the cell and selecting "pick from drop down list..." Then the data options for the cell appear to the left. The first text you see when the cell is selected is just the "input message". It is a little confusing, cause it can just look like drop down options you can't select.
Is that helpful?