I had a similar problem on my Panasonic NN-750W. The black door finisher was removed with a butter knife. The hook and spring were OK, but the spring's lower, plastic fixing was broken off. It was easily fixed with a paper-clip! This was straightened, re-bent and fitted between the lower end of the spring and a convenient loop about 1" below the spring on the door surround. The door hooks now work perfectly!
Dave
February 2010