I had my worksheet working great and then I did something - I do not
know what - and data validation crashed.

My goal is to have 3 columns, each with 20 entries, with the user
entering data into columns A and column B. Column C is the total. If
the user enters data into columns A or B that causes the total in
column C to duplicate a prior entry in column C, I want to display a
warning.

Using Data Validation, I had entered the following formula into column
A as a custom validation formula

=and(countif($C1:$C20,C1)=1,ISNUMBER(B1))

In column B I had entered
=and(countif($C1:$C20,C1)<=1,ISNUMBER(A1))

I was getting an error message, which I had placed into the Warning
section of the error alert. All was wonderful. Then I did some other
things to the workbookt, and came back to this. Now nothing works.