Drop of traffic after massive technical issue

[SharonEKG]

Hello,
since August i am working on a customers website on WordPress who has a costume made theme, back in October  after updating some plugins we had a massive breakdown and the website went up and down and had technical issues for over a month and traffic was completely gone for a while, since we have dropped to about 40% of the monthly traffic the website was getting prior, i was waiting to see if the website will recover since we were getting some traffic and are ranking but that did not happen,

is there a way to tell if there are any code issues or anything that can cause that drop? moz crawler only indicates normal meta description errors but nothing in the code, changing the theme would probably be best solution as a popular premade theme would give a definite answer but that is not possible.

[SharonEKG]

so search in google "site:mydomain.com search"?
i am using google search console for a while and today i fetched as Google and requested to be reindexed, i got 504 404 errors but most of them were fixed or supposed to be fixed for a while now.
no smartphone errors, only for desktop. i did just noticed i got 45 mobile usability errors. and no manual actions.
the mobile website is useable and looks decent but pretty bad to operate, most of our traffic is from mobile....
that might be the problem?

[dmcubed]

See what Google thinks with the site:yourdomain.com search. This will give you all the data they have on your site. Look for errors, missing content and duplicate content. I think that would be a good start. Also use Google Search Console to check for index issues, crawl issues and any manual actions. If a site is offline for a while it will be delisted, a big site is delisted after a few minutes, in my experience the traffic returns after 1 or 2 weeks. To make it more complex there seems to have been a few changes to Google results over the past weeks, may be due to the 'mobile first' index.