Perform a composer update, then composer dump-autoload.
If the above doesn't solve the problem, change the classmap in your composer.json file such that it contains the project-relative path to your php files:
"database/seeds/UserTableSeeder.php" //include the file with its path here
}, /** ... */
and soon after, perform a composer dump-autoload, and it should work now like a breeze!
Hope this work fine!