My moble Steam app logged me out and then refused to let me sign back in. I logged out of Steam on the PC and logged back in (same password) and all was fine. Had to delete the mobile link (or whatever you call the procedure) and re-link it to get it to work but the amount of codes I was sent to activate things was crazy. I got emails AND texts on my phone to do it. Finally it worked and now I feel less stressed.