When you are opening siteurl/wp-admin and it is showing blank page and when you try siteurl/wp-login.php it is showing Error: “Cookies are blocked due to unexpected output” and you are unable to login to your site. This error can occur due to many reasons. Lets debug it to find out what is the issue causing this.
Step 1. If you have access to your cpanel/FTP go to /yoursite/wp-content/ folder and rename your plugins folder to any other name (ex: old_plugins).
Step 2. Now again open your login page siteurl/wp-login.php if it is not showing error and you are able to login to your site that means there is some issue in any of your installed plugins.
Now you can try deactivating your plugins one by one and get which plugin was creating problem.
If above step didn’t work for you then try next step
Step 3. Now go to /yoursite/wp-content/ folder and first change plugins folder name again to plugins and rename themes folder to any other name (ex: old_themes)
Step 4. Now again open your login page siteurl/wp-login.php if it is not showing error and you are able to login to your site that means there is some issue in your active theme.
Step 5. Update your wp-config.php file and set
You will get some error message like (Cannot modify header information – headers already sent by…) it mentions the file name which is causing this issue to occur, generally this happens due to white space or closing php tag ?> open that file and see if any space is left before opening php tag <?php or at end of file and remove them.
You can also check your error log file to check when error started and more details.
Note: Don’t forget to set WP_DEBUG to false again once your issue is solved