Calculation partially not working

I have a large spreadsheet with many formulas. I can no longer add formulas. They just show up as text though I do not include an apostrophe at the beginning of the cell.

In addition, when I change a formula, the formula no long works in that cell. For example, if the original formula is

=A1+B1

and I try to add +C1, I get

=A1+B1+c1

(note the lower case)

I've made sure the spreadsheet is set for auto-calculate.

Yikes.

Make sure the cell with the formula is formatted as General (or anything other than Text) BEFORE you edit the formula.

Or try in the (VBA) imediate window
RANGE("D1").FORMULA="=A1+B1+C1" and see wath's happening