If the outside of your knee hits something while your leg is bearing your weight, you might well injure your medial collateral ligament. It can also be injured if you fall over and your knee falls inwards.
You will feel pain on the inside of the joint, and there may well be swelling and bruising. The knee joint itself might swell, and the whole area will become very tender. Although you’ll probably be able to bear weight on your leg, it will be painful if you try and bend the knee inwards.