Menu disappeared: version 3.5.1

Theme: custom theme built by a past employee
Wordpress version: 3.5.1
Possible plugin associated with menu (but not for sure?): Gecka Submenu

I went to…

Appearance –> Menus –> our “Full Menu” (which ran along the left-side of the site)
–> Chose the page I wanted to remove –> clicked “remove” from options

Then the entire menu disappeared from the site.

We have a second menu which runs along the top of the site which is still there (I was able to add pages to it, but am now nervous to remove one in fear that it will disappear too).

We’re a nonprofit, with no IT support. Any assistance about how we can get our “full Menu” back would be great. Still hunting the forums for answers, but please respond with help. Thanks!