Spinning wheel gyros with mechanical gimbals can exceed limits as the gimbals rotate and two or more gimbals can align or occupy teh same plane. In the case of Apollo space craft ( or any thing that causes the rotation to exceed limits) on gimbal lock the angle transducers lose unabiguous axis inputs - the INS computer can not calculate correct craft orientation or attitue wrt intertial axes ( fixed stars) which is what the gyros are for.
Without attitude correct rocket sequence firing to achieve a given course is impossble.
I think A-13 had to resort to sextant.
Gimbal Angles and Gimbal Lock