HELP my bonsai dried out!?

You're just going to have to wait and see. From your picture it looks like the branches are still alive, so they should re-leaf given some time. If the branches have died but the trunk is still alive, it might put out some new branches. Don't trim the roots-it needs all its roots to recover. Plants are good at taking acre of themselves-if it has roots it doesn't need, they will die. Without any leaves, magnesium won't do any good. The plant uses magnesium in photosynthesis.