New images of Comet ISON have just begun to show up again after quite some time. These new images seem to confirm that the comet is still there, although it looks like it may not be quite as bright as everyone hoped.
Why haven't we seen any new ISON images for so long? The answer lies in the geometry of the solar system. The orbits of Comet ISON and the Earth are such that for the last few months, the comet has been lost in the bright glare of the Sun. The comet is on the other side of the Sun from the Earth, still well beyond the orbit of Mars. And the comet is still extremely faint, so any brightness in the sky swamps the faint light from the comet. As the Earth swings around in its orbit and the comet approaches the Sun, the relative positions of the comet and the Sun as seen from Earth shift, so the comet will become more separated from the Sun and appear in a darker and darker sky. This movie from NASA's Goddard Space Flight Center shows how Comet ISON moves through the solar system:
So when does Hubble jump back in? The first chance for new images of Comet ISON from Hubble will not be until mid-October, when the comet will appear far enough away from the Sun in the sky. Hubble cannot point at anything that is within 50° of the Sun. This seems like a pretty large zone to avoid, but it is kept large enough to prevent any stray sunlight from damaging the telescope's extremely sensitive instruments.