I unknowingly submitted plagiarised work
When the instructor said you could work "together", they meant together with a student that hadn't previously completed the assignment. They expect both students (when working together) to contribute equally towards writing an essay from scratch.

Basing your essay on another student's essay (that you did not work together on) is plagiarism.

The best you can say for yourself is that you didn't realize that what you did constituted plagiarism. The ethics committee might be lenient, however, they might also consider that you should have known what the rules were. To be frank, anybody should understand that you can't work based off material previously prepared by somebody else and pretend it's your own work -- that is the very definition of plagiarism.