When you say "I selected part of the html coding " it isn't clear what exactly (or where) you did whatever you did. In order to help, please be more specific.

Did you edit a page template outside of the CMS? Why can't you undo whatever you did?

I am very new to website maintenance and building. Here's what I did: I have a PC, so I right clicked to view page source, thinking I could just copy and paste the part of the coding that had the countdown timer. Immediately after I pasted it into the html block and hit save, the edit button and dashboard buttons disappeared and i am unable to click on any block to edit! I don't know what to do :/ the home page of the website is completely different that the other pages, AND when i go to the site on my mobile phone, i can't get to any other page on the website. http://www.hvcomiccon.com

Try this. In your browser, in the address bar, type in
http://yourdomainname.com/index.php/dashboard/sitemap/full...
(replacing yourdomainname.com)

Click on the Home Page. In the Pop-Up, select VERSIONS
(like you see here
http://legacy-documentation.concrete5.org/editors/dashboard/sitemap...
& for 5.7+
http://documentation.concrete5.org/editors/dashboard/sitemap... )
There you a version before you did this.

Read about Versions (in 5.7+) here
http://documentation.concrete5.org/editors/in-page-editing/the-tool...

Nothing is working ... I have attached a screen shot of what I see. When I go to sitemap (visit), I brings up the home page with no option to edit or go to the dashboard.

Can I make a new home page? If so, how do I do that?

*Screen shot now attached

You need to remove the offending code, it's a load of 'gobbledygook'!

If you can't get into 'edit' mode on the page to remove, try going back with the 'page version' option and remove.

The screenshot only show me the top of the same I see when going to
http://www.hvcomiccon.com

Are you logged in? It looks like you are not. The above links (to use versions) of course won;t work if you are not logged in. Login at http://www.hvcomiccon.com/index.php/login... first, and then try the sitemap link above and changing the version.