Yosemite: Cannot Music / iTunes - black screen and won't accept password
I'm afraid I've got the same problem.

I restarted the Imac yesterday. The close down took more than 3 minutes. This happend before. I forced the closing down by pressing the power button. After normal start up I could not start iTunes anymore. All other apps function properly. The desktop shifts left and a black screen appears. I can operate the dock and other programms though. I can only get out by forced stop of iTunes.
If I attach my iPhone and start iTunes, a screen prompting me to update it appears (on top of the desktop). I cannot operate it however, as it it is complete dead!

I also tried other option as close full screen, launchpad, ect.

Strange thing is that this occurred about the samen time as my wife synced an iPad which belonged to me before. She was asked to enter ID's of which one was my ID related to buys I suppose. 
I checked my iCloud account. It is still active.

PS!!
I just cracked it! I moved (not copied!) the itunes library to a temp folder.
I restarted iTunes and it immediately came up with the familiar screen. Probably a new library was created. I then imported the info from the temp folder (trough the menu). The new library was updated. I could notice by syncing the iPhone. Also my music is back and available.