Common WordPress Issues and How to Fix Them

I am having a major issue on my Wordpress site and don’t know how to fix it. I had to update the site manually as there was a problem with malware and the ISP shut the site down. I got it back up and running with help from a programmer (paid them of course) but now none of the plugins are showing and it won’t even let me install new plugins. I can see the plugins in the file structure on the server, but they don’t show up in the Wordpress admin panel. And so, part of the site isn’t working as it depends on a couple of the plugins–I don’t even have any editing tools when I go to edit a page.
Please advise.
Thank you.