Hi Bart,
Don't know how you corrupted your file, but from the
description it seems you data is fine until you reenter
data  (F2+Enter would cause a reentry).

If the sheet is really corrupt it would be more a matter of
retrieving what data you can.   From your description it
sounds like someone selected all the cells on the sheet
and formatted them as text.    The numbers remain numbers
and the formulas remain formulas until you reenter the data.

Don't mess with your current workbook,  make a copy of the
entire file and play with the copy.

Install the TrimALL macro found at
  http://www.mvps.org/dmcritchie/excel/join.htm#trimall
be sure to read all of the information described in that
topic and in the topic above it, so that you will understand
what happens when you change format without reentry
and so that you will know what the TrimALL macro will
be doing.

the actual code can be obtained from within
  http://www.mvps.org/dmcritchie/excel/code/join.txt
install the TrimAll macro or copy the entire join.txt
file  into a code module in your workbook.

Instructions to install a macro can be found in
   http://www.mvps.org/dmcritchie/excel/getstarted.htm#havemacro

Don't mess with your current workbook,  make a copy of the
entire file and play with the copy.

In the copy -- Select all of the columns that should not be text, don't
worry about  headers at top of columns (unless they are dates).
Then    Format, Cells, General
next run the TrimALL macro,  without changing that selection.

---
HTH,
David McRitchie, Microsoft MVP - Excel
My Excel Pages:  http://www.mvps.org/dmcritchie/excel/excel.htm
Search Page:        http://www.mvps.org/dmcritchie/excel/search.htm