Marketpress - cart broken after plug in update

From a patron:
"This morning I attempted my purchase again. This time all went well, including being able to get as far as PayPal. However, after logging in, reviewing the purchase information, etc., etc., and clicking "continue" which was supposed to re-direct me to the home page to finalize the purchase that didn't happen. Instead, the screen I was redirected to had the LBCS logo but was blank except for a large "OOPS" and an "Error 404" . "?

Hi - I'm the marketing person for this site:
After installing the WPMUDEV plug in to my site (credit card number entered in good faith) the Marketpress plug in showed an update available. After updating, the following error appeared on the dashboard:

The plugin wordpress-ecommerce/marketpress.php has been deactivated due to an error: Plugin file does not exist.

I need help. We're a non-profit and we can't accept ticket sales for a concert that's happening in 2 weeks.