Unusable Excel Files – Unable To Input

Am using Excel MS Office 2016 (Win10), and cannot get a clean conversion to the .xlsx format. Highlights:

• I can input to a single cell, but then no others on the page until I click out of that page and come back again. Also, all functions are frozen (add/delete row, etc.).
• My file has a prior format history (97-2003). Would like to find a remedial “super convert” function, but have not been that lucky.
• I have performed the two conventional convert actions. In Compatibility Mode, I did the File/Info/Convert path. The second was the save-as method. In both cases the file is transformed to the .xlsx format, but the cell input problem is there.
• I have another file (also .xlsx) where I go Review/Check Accessibility, and it tells me the doc is an older version and I should convert. However, nothing happens when I press the Convert button. And the problem persists.
• Software seems not the issue, as I have other files with no problem whatsoever.

These files seem to have long memories, with no way to clear and operate anew. Frustrating because I have lots of work/data into these files. Any help would be much appreciated.

Thanks,

TheKingfish