Count gives NON-BLANK counts, so your "0" is counted
Sum gives total of non-blank, so 1+0 = 1
If you are using the Summary field Dialog, you may want Total of, not Count of:
But that doesn't solve your "greyed" out. You may check the field options for auto-enter, validation and storage. And perhaps other values have sneaked in between Text and Number type changes?
You can see by my screen shot. I know the greyed out are Text fields.
Thank you. I had to remove the indexes I guess created when the fields was text. This solved the problem thank you.
If it is the real reason, I think you find a bug. Changing field type should change its index without any user interaction.