Why has my phone's screen frozen while it's still on?

I was watching a YouTube video on my phone and I took a screenshot of something that I thought was really interesting. All of a sudden, the screen of my Samsung Galaxy S3 Mini froze up while it was still on, and I can't do anything with my phone now! I can't turn my phone screen off, raise the volume, or even lower the notification bar! The screen is stuck on the YouTube video I was watching, at the very moment I took the screenshot, so what's going on?? I'm a college student, so could it have had something to do with my phone being in the same jeans pocket as my student I.D. for quite a while? Can anyone PLEASE help me figure this out ASAP, so I can continue using my phone normally tomorrow?!?

from Android Forums at AndroidCentral.com - Ask a Question //forums.androidcentral.com/showthread.php?t=772891&goto=newpost
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