While searching for a solution, I found a simple and effective method to recover the lost CSS, and I wanted to share the step-by-step guide with you:

1. Log in to your Hostinger account.

2. Navigate to your desired website.

3. Access the Databases section and open phpMyAdmin.

4. Once inside phpMyAdmin, select your website's database.

5. Go to the Query tab and run the following SQL query:

   SELECT * FROM wp_posts WHERE post_type = 'custom_css'

6. Click Submit Query, and you’ll see a row (or more) containing the custom CSS data you had before.

7. Click the Edit button, copy the CSS code, and paste it back into the Additional CSS section of your WordPress site.

This approach worked seamlessly for me, and I hope it helps anyone facing the same issue.