If you happen to eat a dead mussel or dead mussels then you can become sick and even get food poisoning.
So never eat any dead mussels.
If you have dead mussels you should throw them out and do not eat them because you risk getting sick.
When the mussels are dead the dead mussels meat deteriorates, increasing your risk of microorganism contamination, food poisoning, infectious disease and other health problems.
You can even in rare cases die if you eat dead mussels and you get sick enough.
So if you have dead mussels the best thing to do and the thing you should do is throw the dead mussels out.
The same goes for any other food that may be bad or contaminated.
It's not worth risking your health or even your life over.