HELP....cannot access posts after installing new plugins

I am using WP 2.92 and atahualpa 3.4.01.
I installed PHP speedy plugin and cleanup wordpress plugin. The front page of my site still looks as usual but when I click on a post I get the following error message:

The requested URL /blog/4-tips-for-building-trust-online was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
Apache/2.2.11 (Unix) mod_ssl/2.2.11 OpenSSL/0.9.8i DAV/2 mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635 Server at automarker.net Port 80

I uninstalled and deleted the plugins, but the error is still there. What should I do?

I appreciate any and all input!

Thanks,
Karin.