This can happen when the save is interrupted somehow (e.g. shutting off the computer, shutting down Surfer, or an error was generated during saving) or if the file was saved to a corrupted block on the disk.
You may want to check your backups (if applicable) to see if you have a backup copy that contains the full file. You might also want to consider clicking File | Options, checking Backup files before saving and clicking OK. Enabling that option will create a backup file in case the original becomes corrupted in any way. That may help in the future, but not for the current file.
Updated October 3, 2017