So, you are in a situation where the WordPress system files are damaged, and the last backup was made too long ago. Don't worry if it sounds bad - below you will find detailed step-by-step instructions on how to restore your site in this case.
Step 1 - preparation
At this stage, you need to connect via FTP and download the current copy of the site to your computer.
NOTE: you do not need to make changes to the database!
Step 2 - "Clean" WordPress
Next, you need to go to WordPress.org and download the archive with a clean (no plugins and themes) WordPress.
The unpacked archive will look like this:
Step 3 - Copy via FileZilla
Next, you need to copy all files and folders from the pure WordPress folder to the public_html folder on your hosting via FileZilla.
When prompted to overwrite files use this configuration:
Once the process is done you can check your website in incognito mode of your browser, it should be working and your plugins, themes, and the content will be saved!
If you still find your website not working after doing all the steps, you can restore your website from a backup.