If you created some migration config and then you renamed it later, there is a good chance it is still present in config and it can also break your migration UI for that migration group. To clear that out, check your config with drush cedit find id of config and run this in devel/php or drush php
Drupal::configFactory()->getEditable('migrate_plus.migration.my_migration')->delete();this will clear migration from config and render migration UI back to life.