Am really fed up yesterday with this trouble. My domain and files hosted in Godady, normally godaddy cpanel is very easy to handle, am also very much familiar with this cpanel. Yesterday while i am opening my site i can see this error, instead of loading home page one file downloading to my computer.
If you getting this type of error do check your all files, is there any unwanted files added in your files. If you cannot see any errors then dont edit your files or dont remove any files from hosting. Because this error mostly related server issue.
Contact your server team immediately to solve it and inform them this issue. They will fix it for you. Also clear your cache.
Mostly i face this problem while i am hosting files of any cms site from one hosting to another or while i am making development site to live.