Stored file path of a template

site url: http://modx.bildt-law.de/
Suddenly I have got Error 503 and couldn’t be able to fix. I have search but haven’t got any solution. Is there any solution to get back this site?
Or is there any way to find the code of templates in core file?
I have FTP access.
Thanks in advance.
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Does the file core/model/modx/modx.class.php exist on the server?

Sorry. No
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Looks like you’re missing the entire core/model/ directory, perhaps that was accidentally removed.

As you may also be missing other files you don’t yet know off, I’d suggest you download MODX fresh from modx.com/download (use the same version you have installed; see core/docs/changelog.txt or core/docs/version.inc.php), upload all its files into your installation, and then run the setup to ensure everything’s in place.

I have added “Model” files from Modx package without installation but no solutions.

As Marc told, upload all the files and rerun setup.
You could also try to use this installer:
Bruno17/installer (github.com)

be sure, you use the same version as you had installed before!

If I do so, will I get back my templates, chunks and so on?

Should usually work, if you reinstall it over the old installation.
Be sure, to backup, what you have!

And if you don’t have static elements, they are stored in the db.