If you have blocked an app by mistake or you have noticed that you are unable to access that particular app then you need to check or unblock an app by following these steps:
>> You will find one option “Account” on the top right bar in Facebook. Click on that option.
>> Select the option “Privacy Settings”.
>> Choose the option “Block List”. Click on the option “Editing your list”.
Now check if you have blocked an app, then unblock that parlicular app by clicking on the option “Remove”.