Quick answer: Disable a faulty plugin without admin access by renaming its folder in wp-content/plugins via FTP (or renaming the whole plugins folder to disable all), or by clearing the active_plugins value in wp_options via phpMyAdmin. Then log in and reactivate plugins one at a time to find the culprit.
When a plugin breaks your site and locks you out of wp-admin - a white screen or 500 error after activating or updating one - you can still disable it from outside WordPress. Here are two reliable methods.
Connect via FTP or File Manager and open wp-content/plugins. To disable a single plugin you suspect, rename its folder, for example faulty-plugin to faulty-plugin_off. WordPress cannot load it, so it is effectively deactivated, and you regain access.
If you are not sure which plugin is at fault, rename the entire plugins folder to plugins_off. That disables all of them and should restore the site. Rename it back, and the plugins reappear deactivated so you can enable them one at a time to find the culprit.
If you cannot use FTP, use phpMyAdmin. Open your WordPress database, find the wp_options table (the prefix may differ), and locate the active_plugins row. To deactivate every plugin, set its value to an empty set:
Field: active_plugins
Value: a:0:{}
Save. WordPress now sees no active plugins and the site loads. Back up the table before editing, since the value is serialized data.
Log in, then reactivate plugins one at a time, checking the site after each. The one that breaks it again is your problem plugin - update it, replace it, or contact its developer.
How do I disable every plugin at once?
Rename the wp-content/plugins folder to plugins_off, or clear the active_plugins value in the database.
Will renaming the folder delete my settings?
No - it only deactivates the plugin; renaming back restores it, deactivated.
On SoftSys managed WordPress hosting our team can identify and disable a faulty plugin for you and, with staging available, you can test the fix before touching the live site.