Activating plugin wipes out backend

I’ve been working on this all day. This has happening with two different sites on the same server that worked fine previously. I added a new plugin and the entire backend disappeared. I can’t imagine what I’m doing wrong. I’ve reinstalled a new clean version of WP with an empty database. Everything is fine. I can activate Akismet and Hello Dolly with no problems. I can install wp-e-commerce, but when I activate it, everything goes blank.

On another site, my client deleted Hello Dolly without deactivating it first and every plugin on the front end disappeared.

I’d really appreciate some help. Thank you.