Accidentally deleted appdata folder

Is there a way to restore this folder? Or do i have to completely reinstall everything? (It’s a feasible alternative if all else fails)

Nextcloud version (eg, 12.0.2): 15.0.8
Operating system and version (eg, Ubuntu 17.04): Raspbian 9.9
Apache or nginx version (eg, Apache 2.4.25): 2.4.25
PHP version (eg, 7.1): 7.0.33

The issue you are facing:

I have accidentally deleted the appdata_oc1ady95q3ru folder. Now my NC instance is practically inaccessible.

The output of your Nextcloud log in Admin > Logging: I can’t provide that, as i cannot login.

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

<?php
$CONFIG = array (
  'passwordsalt' => 'XXX',
  'secret' => 'XXX',
  'trusted_domains' =>
  array (
    0 => 'cloud.polonkai.eu',
  ),
  'datadirectory' => '/var/www/nextcloud/data',
  'overwrite.cli.url' => 'XXX',
  'overwriteprotocol' => 'https',
  'dbtype' => 'pgsql',
  'version' => '15.0.8.1',
  'dbname' => 'nextcloud',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'dbuser' => 'nextcloud',
  'dbpassword' => 'XXX',
  'installed' => true,
  'htaccess.RewriteBase' => '/',
  'instanceid' => 'oc1ady95q3ru',
  'trusted_proxies' =>
  array (
    0 => '10.8.0.1',
  ),
  'updater.release.channel' => 'stable',
  'maintenance' => false,
  'theme' => '',
  'loglevel' => 0,
  'mail_smtpmode' => 'smtp',
  'mail_smtpauthtype' => 'LOGIN',
  'mail_smtphost' => 'XXX',
  'mail_smtpport' => '587',
  'mail_smtpauth' => 1,
  'mail_smtpname' => 'XXX',
  'mail_smtppassword' => 'XXX',
  'mail_from_address' => 'nextcloud',
  'mail_domain' => 'XXX',
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'updater.secret' => 'XXX',
);

The output of your Apache/nginx/system log in /var/log/____:

stat: cannot stat '/var/www/nextcloud/data/appdata_oc1ady95q3ru/js/core/merged-template-prepend.js.deps': No such file or directory

first i’d try to create the missing dir(s) manually (don’t forget to set the right perms) - if you’re lucky the contents get recreated automagically. maybe you’ll have to reinstall the apps.
(i’d say it’s worth a try before re-installing the whole instance)