Hello there, i'm running the business site https://episkeyh-oikiakon-syskeyon.com/ (country targeted) for years now and was going quite good with many keywords on first results in Google, until recently...a month ago the performance and hits started to drop dramatically and today they have fallen to zero. Thinking back the only change i've made the last period after yoast's plugin recommendation, is that i changed the permalinks from https://episkeyh-oikiakon-syskeyon.com/ p=123 to https://episkeyh-oikiakon-syskeyon.com/%category%/%postname%/. Also, when i'm trying to inspect any URL inside Google Search Console i get the message that "The URL is available on Google, but has issues". Is this change capable of causing this problem and how can restore my site? Many thanks!
You need to REDIRECT old URL to new URL, otherwise you will lose all your rankings. You can do this with htaccess. Something like this might work: RewriteRule ^/(.+/(.+))$ index.php?cat=$1&p=$2 [L]
Google automatically removes the 404 URLs. Your old URLs are now 404 and hence removed from SERPs. You should have redirected the URLs from the old to new permalinks.
First, redirect the old URL to the new URL to restore some of the visibility from earlier, apart from that you can do some new keyword research to grab the attention of your targeted audiences.