According to Jewish sources, an empty room, possibly with a small stone in it.

Talmud, Masechet Yoma 21b:

...in five things the first Sanctuary differed from the second: in the ark, the ark-cover, the Cherubim, the fire, the Shechinah, the Holy Spirit [of Prophecy], and the Urim-we-Thummim [the Oracle Plate]? — I will tell you, They were present, but they were not as helpful [as before].

The first opinion, that the ark was missing is also supported elsewhere in the Talmud. Later on, same section 53b. The Mishnah says when describing the ritual services of yom kippur:

MISHNAH. AFTER THE ARK HAD BEEN TAKEN AWAY, THERE WAS A STONE FROM THE DAYS OF THE EARLIER PROPHETS, CALLED THE SHETHIYAH, THREE FINGERS ABOVE THE GROUND, ON WHICH HE WOULD PLACE [THE PAN OF BURNING COALS]....

And the Gemara relates two opinions, one that the Babylonians took it, and another that it was hidden somewhere, and then never recovered.

Now, what the Babylonians and all the other conquerers who looted the frist temple saw when they entered, is a different question.