Fix PHP error "Cannot modify header information - headers already sent "

I have a small PHP form to PDF that used to work fine. Nothing in the code has changed. Now when you attempt to run the file, the header is missing (logo, my business info etc.. about 2inches of info) and I get an error which displays what line is having a problem and the output problem. ALSO: I have already checked if there is a space after php> or ?> -- everything is fine and the code hasnt changed in months. The form simply stopped working. Again - this file was working just fine a week ago. Also, I have another form which uses the exact same code and it works fine. So I am confused and need assistance. I request that once fixed, inform me what the trouble was. You will not get server access. You will have FTP access to the folder only. If you need server access I will share my screen with you and you can tell me what you want to see. Again, nothing was changed in the files or the serve r- the form simply stopped working properly. If you have read this far, DO NOT send me your resume or life story. Place the words "CANNOT MODIFY HEADER INFO - I CAN FIX THE PROBLEM" <<< sorry to be blunt, but it seems no one reads the details.