You don't mention whether the missing drive is found on boot-up, or whether there are any lights shown on it during boot-up so I assume not. Check to make sure that the data cable is still firmly fitted at each end and that the power cable is also firmly in place. These can easily become dislodged during cleaning. To be safe, take each end of the data cable off and the firmly replace it, making sure that its the right way round. Then do the same with the power cable.