Horror Struck

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"On mounting a rising ground, which brought the figure of his fellow-traveller in relief against the sky, gigantic in height, and muffled in a cloak, Ichabod was horror-struck on perceiving that he was headless!..."

—The Legend of Sleepy Hollow, Washington Irving

A character has been called to a small town to investigate some supernatural phenomenon. Our intrepid hero subscribes more to logic than superstition, though, and he is confident that the disturbance will prove to have mundane origins. The locals tell him tales of this horrible monster, and he laughs them off as the products of an overactive imagination. With this knowledge under his belt, he bravely sets out to catch the miscreant.

This ends when he actually SEES the monster—and finds that it is just as advertised, and worse. He sees the eerie, glowing intelligence in its eyes, smells the rotten meat on its breath, gets smacked around a bit, but survives, running back to safety to hide under the bed and refusing to come out because it's real it's real OH MY GOD IT'S REAL... even though that's what everyone's been telling him up till now.

Inspired by the protagonist of the Legend of Sleepy Hollow and in particular Ichabod Crane's portrayal in Tim Burton's Sleepy Hollow, this state is what results when mundane presuppositions come crashing against the supernatural reality, and the character in question just can't handle it.

Differs from Heroic BSOD in that this is not necessarily a result of emotional trauma.

Examples of Horror Struck include:

Film

Sleepy Hollow, natch. Not so much in the original legend, since in that version a) Ichabod believes the story, and b) it was a hoax.
The Blair Witch Project.
In The Mothman Prophecies, John Klein, a journalist, investigates a mothlike creature roaming in Point Pleasant, West Virginia. The usual glowing eyes, mauled livestock, scratched tree bark reported from townsfolk and cops...So those phone calls and dreamlike predictions coming from this so called Mothman? They're real. I know what's in that drawer. Chapstick.
The Brothers Grimm.
In My Name Is Bruce, Bruce Campbell follows this trope to the letter. His frenzied flight includes shooting at the men who came to help him slay the beast, stealing a bike from a little kid,and carjacking an old lady before he finally makes his cowardly escape from the town.
Grave Encounters features the crew of a Ghost Hunters-style reality show, who are filming overnight in a mental hospital but express skepticism that any of the ghost stories about it are true. Of course, once they are in lockdown, shit gets real...

Literature

The Stephen King short story and subsequent movie 1408 has several of these - moreso the movie, because there the protagonist has just about given up on the idea of genuine hauntings.
Just about every HP Lovecraft story.

Theater

Horatio, faced with the ghost of old Hamlet.

Video Games

While it's up to the player to decide precisely how to react, this is conveyed pretty well in Nethergate when you find out just what's living in the caves you're investigating. It's subverted later on, though—facing an assault by hordes of ghosts, you shrug it off through sheer Badassness.

Web Original

Present in quite a few works in The Slender Man Mythos: especially in later blogs, where people start assuming that Slendy's just an urban legend, and his appearance in their lives is problematic. For particular examples, see Seeking Truth and Everyman HYBRID.

Western Animation

Scooby Doo has seesawed on this point in more recent adaptations, showing Freddy pulling on a mask to no effect, and everyone then running away in fear.