I am importing data from a text file and reducing it down to 2 columns; one
with names, one with numeric values. I save the file as an Excel workbook
and then proceed to edit the data. This editing involves sorting, performing
some deletions, re-sorting, etc. Sometimes for a reason entirely unknown to
me, the term [GROUP] appears at the end of the file name. When [GROUP] is
invoked, I can't sort the data.

What causes the status of my file to change to [GROUP] and how do I get the
file back to a 'normal' Excel worksheet (so that I can perform sorting on the
data)?

It may be caused during the course of clicking around among and between
tabs that you accidentally press the Shift key. You can make it happen
on purpose by pressing Shift and clicking a different tab: note that
both tabs are highlighted and you receive the [Group] notation. When
[Group] is invoked, any change you make on one tab will be made in the
grouped tabs as well, so use caution!

De-invoke [Group] by right-clicking a grouped tabname, then select
Ungroup Sheets.