D300 cards not accepted chr message

try the last card you had in it before this problem occurred sometime the processor in the camera doesn't realize the disk is gone sort of like your computer once it starts reading a disk and you remove it half way threw the load. Well usually before yu can go any further on the computer you must put the **** back in and close the program out. This issue is usually case on cameras when the disk is removed before the camera is turned off.