I activated some plugins for my website and now I can't access my website or the wp admin page. error 500
You answered your own question. Not all plugins are safe to use and valid, or well programmed. One (or more) of the plugins you are using is broken, and without
error_reporting you can't tell which one. Open your error logs to find out.
Regardless of the error logs, remove the plugins you installed and add them back one by one until you find the culprit plugin and skip using it, and possible let the developer know what's happening.
Unfortunately since your
wp-admin isn't loading, you will need to do this via FTP or your hosting account (WHM, cPanel, etc.)