reset of website assistance

hello.
i please need some advice to reset the website that i have problems with.
i installed the plugin “idle logout” but it doesn’t work and makes my site crash and the time setting that doesn’t change.
if i go to the toolbox and select the function “system reset” does it delete everything including the theme or only parts of the theme?
all i need to delete is the plugin but i can’t even get it to deactivate to delete the plugin.
please give me some advise on how to proceed and what will be deleted if i reset the site.
thank you.