The shuttle launch is currently on a 72-hour minimum hold.

It was a leak of hydrogen gas, not LH2. The leak occurred at a weld-line, where two sections of pipe were joined. The age of the pipe dates back to the Apollo era, so they are quite old. In spite of the age of the pipe, these types of failures are common for near-cryogenic pipe welds. The reason it appeared to be a very serious leak is because the weld-failure propagated rapidly around approximately 3/4 of the circumference of the 16-inch diameter pipe. The pipe was double-walled with no liquid was flowing through it. By design there was already gas flowing through the gap between the inner and outer walls. So the leak being visible on the outside does not mean that there was a fracture on the inner pipe. This design has something to do with the near-cryogenic operating conditions. The leak occurred at a location that did not put the orbiter at risk; the hydrogen gas dissipated rapidly and never approached the 4% H2-in-air concentration needed for combustion. It was also located on the side of the mobile launcher and did not put any personnel at risk. So sayeth NASA.

The next earliest launch is Sunday.