Installed & it crashed my site - then deleted the folder & site still down!!

I installed the plugin on a non mu site (didn't see the MU only thing!) and I got the message (that a lot of others seem to have gotten 'The Signup Password plugin is only compatible with WordPress Multisite.'

So I jumped into FTP to delete the folder, and it is completely gone, but the message remains and I can't access front or back end. Please help!