Warning: cannot modify header information error

Hi,
I just joined the forum and need help ASAP so don’t have time to search all posts. Forgive me if this has cropped up before :). I was editing my site and everything was working fine and then suddenly when I went to create a new page there was the warning error cannot modify header information. I made the page and it saved it after giving me the warning but when I tried to save/publish another page I was directed to a page that simply had the warning message on it and wouldn’t go back to any other page. The page that was saved has also disappeared. When I tried to login again the warning appears on the top of the login page and takes me to the same page with the warning repeated several times. I’m using the Femme Flora theme. The only thing that may have caused it was putting a widget in the footer-everything was working fine until then, but I can’t get back to anything to remove te widget. The site itself, not using the admin login is fine except it doesn’t have the recently created page on it. I hope this was clear enough any help you can offer will be greatly appreaciated. TIA,
Gaynor