I do not know when musing will bring back that upvote button again to its front end. But for anyone who is so concerned about this upvote button in musing can do so from steemit page. Although it will be a bit lengthy way to cast a vote to musing question or answer, but if one is so concerned about voting, then I think this lengthy way to vote is also okay for him.
If you can not see the option of upvote button in musing front end, it is always there in the backend and you can see that from steemit page. So you can still cast a vote to a musing question or answer.
Actually I was also surprised when i woke up this morning and I can't find the upvote button. I actually thought maybe it was my phone or browser settings.
If it was an error from the musing.io group, then I am sure it is going to be fixed very soon because I know every error can fixed so I am sure they gonna work on it to be fixed.
Also if it was the musing decision to remove it, then I am sure they have their own reasons for it which one know will have much more benefits to musing.io
Generally,I am not worry about the upvote button because I will just continue doing what I love most on musing which is answering of questions.
All I know whether it is an error from the musing or it was their decision to do so, then they only best.
I'm aware that the upvote button is already back but I just want to drop here a comment by @jonching (creator of Musing) as to the reason why the upvote button was missing hours ago.
From the looks of it, it seems Steemit Inc made some changes that might have affected some functionality that Musing uses and it wasn't Musing's intention to remove the upvote button for hours. :)