When the dashboard is also unreachable, the fastest safe route is WordPress recovery mode rather than renaming folders blindly. WordPress usually emails the site admin an error notice with the subject line about a technical issue — that email contains a recovery-mode link that lets you log in even while the error is showing. Once you are in through recovery mode, go to Plugins and deactivate everything, then reactivate them one at a time, checking the site after each one; the plugin that brings the error back is the cause. If no recovery email arrived, use your hosting control panel’s file manager and rename the wp-content/plugins folder to plugins-off — that disables all plugins at once and normally brings the dashboard back so you can sign in normally and repeat the one-by-one test. Only if the error continues with all plugins off should you look at the theme: switch to a default theme such as Twenty Twenty-Five the same way. Working in that order (plugins first, theme second) finds the culprit without risking your content, and everything you deactivate can be reactivated, so nothing is lost by testing.
