Yeah well, a few days ago my .htaccess file's RewriteRule which re-directs almost every page on my e-commerce website stop working, and all I got now is a 404 error page. I just can load the frontpage.

I have e-mailed to technical support but is SO slow, and my web keeps inoperative. Any help? I haven't touch the .htaccess in days so I think the problem comes from the server.

This was caused by an update on the way that mod_rewrite's work.
I saw that you already fixed this on your hosting package, but for future reference, you would want to ensure that
RewriteBase /
is added to your .htaccess file right after the line
RewriteEngine On

If your domain is pointing to a subfolder of /htdocs, then you would want to use
RewriteBase /subfolder

I hope this information is helpful.

I am attempting to add an .htaccess file to my root directory. I am able to add it as a .htaccess.txt file, but am not allowed to remove the .txt file extension (File Type Not Supported Error) is presented when I either try to upload the file as such or rename it when there.
Any work around on this?

Are you trying to use this on an Ecommerce package? This will only work on a hosting package. Ecommerce doesn't use .htaccess.