im gonna make a post here in non caps so i can help people remain calm
everyones theme has been erased and replaced with what seems to be a default theme, and everyones html has been erased
some themes still work apparently? but mostly everyones has been erased
you have not been hacked and your blog has not been purposefully sabotaged
its best to wait patiently until tumblr fixes this
and when that happens, you can recover your theme here https://www.tumblr.com/themes/recover but right now that wont work
so just wait patiently and calmly until this is resolved and do not panic!