I assume you mean the "Ark of the Covenant".
See here for the Jewish Encylopedia version.
There are two opinions, either it was captured and taken to Babylon, (Yoma 53b) or hidden away in the Temple. The Mishnas in Shekalim are interesting:
Shekalim 6: Mishnah 1: There were thirteen horn-shaped chests,'
thirteen tables, and thirteen prostrations in the Temple. Those of the
House of Rabban Gamaliel and they of the House of R. Chanania the
Prefect of the priests used to prostrate themselves fourteen times.
And where was the extra one? '-Opposite the woodstore, for thus was a
tradition among them from their ancestors that the Ark was hidden
Mishnah 2: It happened once that a certain priest was occupied and
noticed that some flooring- was different from the rest. He went and
was telling it to his fellow, but before he could end the matter his
soul was departed from him.' And thus they knew definitely that there
the Ark had been hidden.