My CSS suddenly doesn’t load anymore [closed]

You should remove an additional } on line 18. Try the following

<?php

if (!function_exists('understrap_scripts')) {
    /**
     * Load theme's JavaScript and CSS sources.
     */
    function understrap_scripts()
    {
        // Get the theme data.
        $the_theme = wp_get_theme();
        $theme_version = $the_theme->get('Version');

        wp_enqueue_style('slider-slider-styles', 'https://cdnjs.cloudflare.com/ajax/libs/slick-carousel/1.8.1/slick-theme.min.css');
        wp_enqueue_style('lity-style', 'https://cdnjs.cloudflare.com/ajax/libs/lity/2.4.1/lity.min.css');
        $css_version = $theme_version . '.' . filemtime(get_template_directory() . '/css/theme.min.css');
        wp_enqueue_style('understrap-styles', get_stylesheet_directory_uri() . '/css/theme.min.css', array(), $css_version);
    }
} // endif function_exists( 'understrap_scripts' ).

add_action('wp_enqueue_scripts', 'understrap_scripts', 10);