WP 8.1 Lumis 735, Sound non responsive

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So, I have a Nokia Lumia 735, I'm a Spotify premium user, I've never had a problem with the app before until today...

So, I'm playing music on my phone as normal and go to turn the volume up, the volume on my phone is going to max but the music isn't changing volume? I've closed the app, reinstalled it, closed my phone, tried headphones, nothing is working?? Every other app is responding as normal, can someone help please??