There seems to be some conflicting information about whether it's simply an engine (propulsion) problem or whether the problem was caused when the ship hit something. Here's the AP story:

NA-GEN--US-Queen Mary 2,0200
Queen Mary 2 returns to port with motor problem
PORT EVERGLADES, Florida (AP) - More than 2,500 passengers were
stranded aboard the Queen Mary 2 on Tuesday after the ship had a
problem with one of its propulsion motors, Cunard Lines said.
The ship had a "problem with one of the ship's four propulsion
motors" while leaving Port Everglades, but no one was injured,
Cunard said.
The ship's crew reported hearing and feeling a shudder as the
ship was leaving the port, Coast Guard spokesman James Judge said.
Preliminary tests indicated the ship may have hit a submerged
object, Judge said.
The Queen Mary 2 will undergo an inspection by the Coast Guard
before it is allowed to return to port.
The ship, the world's largest and most expensive cruise ship,
had stopped in South Florida as part of a 38-day trip around South
America.
Queen Mary 2 will lose the title of world's largest passenger
ship when rival Royal Caribbean International launches the Freedom
of the Seas in May.
Cunard is owned by Miami-based Carnival Corp., the world's
largest cruise operator.