So he died of shock.
2 seconds sound clip from the The Mummy (1932) movie soundboard.
You can hear this line at 00:29:58.440 in the Blu-ray version of the movie.
Quote context
[...]
- There was something about her head...
- Imhotep?
- What was this Ardeth doing in the museum?
- Looking at the mummy, just at closing time.
- Hello. What? Il est mort? Tout de suite, effendi.
- Come. A museum guard found dead in the room of the princess.
- The cause of death, I not find nothing.
- Looks like an attempted theft, but nothing stolen here could be disposed of.
[...]