Formatting changes the display, not the underlying value. In particular, it
will not turn text into a number, so you not have ruled out Luke's
suggestion, though a different explanation seems more likely to me.
Can you be more specific when you say that "formulas aren't working"? Are
they displaying the formula itself, or an old result that does not update
properly when cells are changed that are used in the formula?
If formula cells display the formula itself instead of the result, what
happens if you select multiple cells that contain at least one such formula,
select empty cells, and Paste Special|Values (under the Edit menu in pre-2007
versions [try Help if you have 2007]). If you get the result(s) instead of
the formula(s) in the pasted cells, then you have probably accidentally
placed Excel into Formula Auditing mode. In pre-2007 versions either
'Tools|Formula Auditing|Formula Auditing Mode' or
CTRL + ` (grave accent) will toggle the auditing mode on or off (CTRL + `
may still work in 2007, but I have no way to verify).
Jerry