You need to find the offending blocked domain and whitelist it (if you wish to – it could be a major ad serving domain). To do this in a browser navigate to the web interface by typing pi.hole/admin and click on Query Log. You may be able to see the blocked query or queries immediately or you may need to open a new browser tab and try to go to the website again. Once you have found the domain that is being blocked simply click Whitelist on the right hand side, wait a minute or so and reload the website. Repeat as necessary until the website loads.
You can view and delete domains you’ve whitelisted by clicking on the Whitelist link on the left hand side panel of web interface.
If you have added a lot of additional blocklists and continually have issues with websites not loading you may need to review which lists you added. You can do this under Tools -> Query Lists. From here you can search for the websites that are being blocked and find which blocklists are commonly causing problems.