Definition
Existential frustration is a state in which the will to meaning is thwarted. A person cannot find meaning or loses it after a loss, disappointment or life change.
Do not pathologise
Frankl warned against treating doubt about meaning as illness. The question of meaning is an expression of humanity, even a sign of maturity. A doctor who answers such a question only with medication confuses spiritual distress with disorder.
From frustration to neurosis
When frustration lasts long and remains unresolved, it may turn into noogenic neurosis.
Sources
V. E. Frankl, Man's Search for Meaning; Wikipedia: Noogenic neurosis.