This happened to my server after I made a mistake in the configuration file.
I didn’t have a backup.
I managed to solve it by removing all the settings I had entered and leaving them as they were originally in the system and restarting.
The configuration.yaml file looks like this:
# Loads default set of integrations. Do not remove.
default_config:
# Load frontend themes from the themes folder
frontend:
themes: !include_dir_merge_named themes