If there are still multiple cells selected, it could be that the cells are actually merged. You can unmerge the cells, if desired, or simply accept that you can't select individual cells in the merged range. If the cells are not merged, it could be that the extend mode is currently turned on.

How do I stop multiple cells from selecting in Excel?

To stop extending or adding to a selection, press F8 or SHIFT+F8 again.” If this is happening to you, you may have hit the F8 Key accidentally.

Why is Excel grabbing multiple cells?

This can be caused by the wheel on your wheelmouse being stuck in a down position or by hitting the F8 key. Either of these actions turns on "Extend selection". You'll likely be able to tell right away if that's the cause.