Hi Mark -
well, I know for a fact the skin won't crash the site - but I can tell you for a fact that uploading a container as a skin can and does make the site not visible.
It's easy to fix - but you'll have to use ftp to do this -
Download the default blue skin from the website.. and rename it to the name you set for the admin skin and upload and overwrite that file.
The skin will appear.
Just check you didn't accidently upload a container. I've had thousand and thousands of downloads of this skin so I'm sure it's ok, but we do often see mistakes like this - even I've been known to upload to the wrong place.
Give that a try - but once again - you can't kill your website with a skin.