plugins cause problem with 3.0.1 upgrade

Tried to upgrade to 3.0.1 from 3 via the dashboard upgrade. Caused my website to disappear with error message “Briefly unavailable for scheduled maintenance. Check back in a minute.”

Website host support brought back site, but only by disengaging plugins. So now I have my site back, but without plugins like shopping cart and contact form.

Can anyone please suggest what I can do to get things back to normal?