All Admin pages giving 404 Page Not Found

I am new to WordPress, but I have a client who has an old website he needs updated. When I log onto his website using admin login all I get is 404 – Page Not Found on every endpoint. What do I need to do in order to access his site and edit things?

Example:

wp-admin/admin.php?page=theme-options

https://preview.redd.it/ejbf16pxb5g91.png?width=1192&format=png&auto=webp&s=9fab215b36790fb64e8027569098ce8a2b62089e

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