The AP will capture the altitude. The speed will increase. The AP is no longer controlling speed, control to a speed limit is a function of the AT at this point. If its inoperative then there is no speed protection, the crew must manually select the required thrust. If they forget, the overspeed warning should give them a clue.
Pitbull's golden rule for operation without autothrottle:
IF THE PITCH MODE CHANGES YOU NEED TO MOVE THE THRUST LEVERS.