It is usually due to spaces, new lines, or other garbage at opening/closing PHP file <?php tag or after a closing ?> tag in wp-config.php. It can be other file too so need to check error message as it may list file name where the error occurred.
You can either upload error file from the backup or edit erroneous file. To edit the file one need to check below mentioned points:
1. Download the file mentioned in the error message.
2. Open that file in a plain text editor
3. Check that the very first characters are <?php
4. Check that the very last characters are ?>
5. Place the cursor between the ? and >
6. Now press the DELETE key on your computer and keep that key pressed for at least 15 seconds.
7. Now type > and
8. Save without pressing any other key at all.