Class 'Monolog\Logger' not found in ConfigureLogging.php

I don't know why and when it happened. when I refresh my project home page, It just show this message:

PHP Fatal error:  Class 'Monolog\Logger' not found in /home/mrdp/Code/firstapp/vendor/laravel/framework/src/Illuminate/Foundation/Bootstrap/ConfigureLogging.php on line 48

and now when I want to update composer, it still show this massage

Your answer will be helpful :) Thanks

via Chebli Mohamed