500 Internal Service Error

I dont know why my last thread shows nothing. sorry for the duplicqate post. long story short, i tried to use Yoast plugin and I got a 500 Internal Service Error now. one suggestion was to go through the file manager and delete it. i did, still no luck. it also said to look in the error log and there are a bazillion errors in there right now. but i cant find anything to explain what to do in a clear, easy to understand way. Everything assumes you know what you are doing. i dont, i had never seen an error log before now. so the plug in is gone and all the permissions are 755 or lower. please help. this is one of the errors i see in the log

[Sun Jul 29 20:39:55 2012] [alert] [client 93.197.87.65] /home/proboxin/public_html/.htaccess: RewriteRule: cannot compile regular expression ‘^([^/]+?)-sitemap([0-9]+)?\\.xml$’\n