Liev Schreiber is opening up about suffering an actor's "worst nightmare" during a performance on Broadway.

The 56-year-old actor has been starring in Broadway's "Doubt: A Parable" since February and ahead of one show, he unexpectedly suffered from acute amnesia.

Schreiber detailed the incident on "Late Night with Seth Meyers" Thursday, telling the host, he had a "terrible headache" ahead of the show and knew "something was really wrong" when he ran into his co-star Amy Ryan backstage and couldn't remember her name.

He also couldn't recall common actor colloquialisms such as "break a leg" even though he "said it at least 30 times by that point."

The "Golda" actor said he mentioned something was wrong to the stage manager, but they quickly called places.

"I go on, and I do about the first six or seven lines of the play, sort of automatically. I realize I'm not connected, so I decide, let me try and get connected again. And the minute I do that, it all vanishes. The play is gone from my head," he recalled. "I'm looking out into a dark audience. I know I'm in a play, but I don't know what play I'm in. I look down at my clothes, and I realize I'm a priest, which is helpful, but not giving me any lines." 

Schreiber's understudy had to take over midway and his friend who is also a doctor rushed to his dressing room with a "terrified expression."

The actor said he was convinced he had a stroke, but MRI results were "perfectly fine."

"I find out ultimately from a neurologist I had something called transient global amnesia," Schreiber said.

What is transient global amnesia?

Transient global amnesia is a "temporary, anterograde amnesia with an acute onset that usually occurs in middle-aged and older individuals" and often caused by "strenuous activity, high-stress events, or coitus, but it can be seen with migraines as well," per a StatPearls dictionary in the National Library of Medicine. 

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The condition typically resolves in 24 hours, but the "Ray Donovan" actor joked that "as a typical sort of Jewish hypochondriac person" he was "convinced that I had a stroke, and they just didn't find it."

Schreiber slept it off and by morning his lines were back. "I go to sleep, I wake up, I remember the whole play. I call the theater and I say, ‘I’m coming up to do the matinee. And they're like, ‘No, no, no. Your doctor said you have to stay away until Tuesday.’ And I was like, ‘No, I need to know that I can do the play,’" he said.

"I never had another problem with it. I was embarrassed and thought everyone would think I was lying and taking a night off from the theater," the actor said.

He also joked that he was bummed out he got "transient global amnesia" from a migraine instead of rigorous sexual activity.