I'm using a workbook that was setup by a different user. When I double click
into ANY cell that contains a formula, the formula stops working and is then
changed into text in the cell. When I try to enter a new formula (even VERY
simple one's) the only thing that appears in the cell is the formula text! I
have NO clue. I've made sure the autocalcuate is turned on.

This is because the cell format is set on "text" and not on anything
else (usual: standard")

maybe you can change it for every cell:
go into the menue format Style
and change this format for the "normal" (or regular) style