What does underwent mean?

undergo: to endure

underwent is just the past tense of undergo.

if you were to use "undergo" you would say

"She no longer wanted to undergo such pain, and so, she took her own life"

or

"little did she know, she was about to undergo such a change that no one could prepare for"

in this case, "underwent"

would be used like...

"Because of this dissociation, she underwent a change both mentally and physically."