It sounds like you are making use of a Multi-Line text field that is making use of Log Editing. I would suggest going into your field settings for that field and checking to see if the option 'Log the edits to this field, and show them on forms'. If that is enabled it doesn't allow for older entries to be edited as they are now logged. That option would need to be disabled long enough for you to edit it and then re-enabled. Do be aware that while disabled any edits others make to that field will not be logged in the same way until it is re-enabled.