I agree with pumpkinsherbert,in reference to shaking the tank.I hate to say this,but hes probably dead or close to dieing

..if hes alive,take him to a vet AS SOON AS POSSIBLE,if hes not.Im sorry for your loss...
If it comes to that,the best thing to do is take care of your remaining turtle and take it to a vet ASAP,theres a chance that whatever disease your other turtle had your remaining one might have it.