Thank you for the replies. Unfortunately I've tried multiple hard resets (i.e. holding both the power and home buttons simultaneously), as well as recovery mode. I've also ensured the phone has had an opportunity to charge, although it had a full charge prior to my issues. Under any scenario the phone is unresponsive.
Somewhere I read a few others had their phone lock up and die (black screen, no response) when using iTunes via their phone, which was the last thing I had done prior (as in 20 mins) to it not working. Don't recall a workable solution but I'm all ears.