Textbox Lost Functionality - '=EMBED("Forms.TextBox.1","")' missing

Morning All,

I created a worksheet for a friend that had a textbox above each column header.

The data entered into these textboxes was used to sort the data below it.

Worked fine for a week but then stopped working. Looking into it, the textboxes appear to have lost the functionality and behave more like pictures (unable to click in to enter text, right click does not provide 'Properties' or 'TextBox Object' options). I note that '=EMBED("Forms.TextBox.1","")' is missing from the formula bar - it's now blank. To be honest I don't understand what it does, but given that appears to be the main difference between one that works and one that doesn't, I'd say that might be my culprit.

Questions:
1) Can I re-embed the textbox (pasting =EMBED("Forms.TextBox.1","") into the formula bar doesn't work) or re-instate its functionality somehow or do I need to redo them all.
2) Any suggestions what would cause that to happen? I didn't have protection on it last time so possible the user could have deleted that line (they say they didn't). Another suggestion was a collegue opening it on a Mac?

Appreciate your thoughts,

Joe