Were you shooting with the lens set to the same aperture on the D700 as you were using on the D300 when the problem occured?
Try stopping the lens down to f/22 and pushing the DOF Preview button on the D700 several times. If the aperture blades don't stick the issue is more than likely in the D300's stop down mechanism. Try doing the same test with a different lens on the D300 and see what happens. If the aperture sticks closed, the issue is the cameras stop down mechanism. If it continues to work, it is possible that something got jammed for some unknown reason and the problem resolved itself. Good Luck and Enjoy your Nikons!