why does scrooge think something is wrong with the clock?

Scrooge thinks it should be daylight yet it is still dark outside,

"Why, it isn't possible," said Scrooge, "that I can have slept through a whole day and far into another night. It isn't possible that anything has happened to the sun, and this is twelve at noon."

The idea being an alarming one, he scrambled out of bed, and groped his way to the window. He was obliged to rub the frost off with the sleeve of his dressing-gown before he could see anything; and could see very little then.