I had created one wordpress site. It was working nicely but now it showing the error as follows :
Warning: Cannot modify header information - headers already sent by (output started at /home/**/public_html/wp-content/themes/**/includes/classy-options-init.php:1) in /home/*/public_html/wp-includes/pluggable.php on line 866
What is exact problem please help me.
Read up on how to go about troubleshooting this extremely generic error message:
Again, this message gives us virtually nothing to work with. Aside from the line # in pluggable.php, the question might as well read "this doesn't work". Try googling for "pluggable.php on line 866" to see some likely causes, since that's all you have to go on at the moment.
Additionally, take a look at your web server (apache?) error logs, and see if you get any more info out of them.
Finally, take a look at other questions involving pluggable.php on SO. They may or may not be a dupe of this issue. For example:
Cannot modify header information - headers already sent by... Wordpress Issue