Cannot load individual posts or pages

All was running fine for a year or more, then…
In Settings/General;
line: WordPress address (URL): data “http://mydomain/blog”
which holds bothWP install and all posts & etc
next line: Site address (URL): read same as above.

BTW WP, after that line the “Enter the address here if you want your site homepage to be different from the directory is most confusing!

WP kept incorrectly and repeatedly gave error message and would not let me Save. So finally I made a fatal mistake and removed the “/blog from the WordPress address line. Of course then that broke the entire site to such extend that login was impossible, and I went thru hell with MySQL, finally found the line and bingo, I got back into myadmin.

The above is just “history”, the problem remaining is merely that my blog opens with (10) articles, but single articles return a 404-type message. I finally installed a different theme, and some single articles came to open, but not nearly all of them. Please just tell me what I need to do to fix that? Thank you muchly!
running XP,Firefox or Chrome.