Ok
Next, if the button wasn't the thing, lets try to see if the motor has power.
I've not done this with the Magnum but the Eheim folks talked me through this so let's try it.
Take the filter apart. Remove the impeller. Straighten out a paper clip and put it into the impeller well. Plug the power in and turn it on, the paper clip should vibrate -- that means the motor is working. If it is not, check parts prices but probably best to dump it, I'm not sure.
Next, look at the impeller. When you turn the blades, they should not go all the way around, they go part way then stop. If it goes all the way around then replace the impeller, many large pet stores carry these, not cheap however. The shaft should be straight, the impeller should be magnetic. It should not have any particularly shiny spots on it.
After that, I'd check the assembly again. But even if the two gaskets were out of place it ought to run, loudly. If the cage the impeller is in is not put back right, if it is crooked, you will bend the impeller and it might not work or will run loudly.