By definition, a “Internal Server Error” is a complete failure of the server caused by the execution of erroneous code. There are many-many things that could cause such an error, so it’s difficult to give you a good answer. Consider what changes were applied to the server or the website before the error appeared.
Taking in considering the previous conversation, I think your website was wrongly integrated with the plugin that you deleted. Causing the entire application to throw an unrecoverable error that your web server cannot handle, and so it decided to throw a “500 Internal Server Error”.
You should revert any hard integration that your website have with that plugin.
If there is no hardcoded integration, it is probable that during the deletion something got modified in the code and a syntax error was introduced. You will have to inspect the “error_log” files and fix whatever error was thrown by your web application.
Let me know if you need more information.
