Excel isn't responsive to simple actions

Hello,

I have a comprehensive workbook that i've been using for a year now. Lately, i've been experiencing difficulty executing the simplest of actions. Some of which are:

Selecting a cell and the cell isn't highlighted. (i see the cell change in NameBox, but doesn't allow editing)
Changing formatting of a cell.
Deleting a worksheet tab.
selecting a range (i see the cell or range change in the Name Box, but doesn't allow editing)


Other notes:
I have had macros in this book, but have since deleted them all.
Workbook and/or sheets are unprotected.
I am the Creator of this workbook and no one else has access.

Current work around.. I can change worksheets, click in a blank cell (i then see the highlight), hit space bar the backspace, the return to original tab and i can perform modifications to cell. However, i still am unable to delete the sheet. Also, i have to go through this exercise for every cell i wish to modify. I'm also experiencing this in other workbooks. I'm an above average excel user (not a pro by any means). Any suggestions are appreciated. I am unable to post the workbook due to confidential and PHI data.
Thanks,