My phone does this also. Ive had it more than 14 months and it has done it 15 or 20 times.
To force a restart hold the Home button and Lock button simultaneously for several seconds-- sometimes 15-30 in fact. It will usually reset and power up. Once or twice it still hasn't worked but I was still able to force it using this method after several tries. Only once has it taken an hour to reset and that was a very hot day.
I am not sure why it does it though. I tried updates and deleting alot of pictures. Mine is a 32GB model so when I dont keep up I can easily use 25GB, but regardless this has still happened with much fewer pictures and movies on the device. I am interested to see if anyone has anything new on this. For now, hopefully that will help you get it back online quicker.
Hey gemstone 60. It sounds like your iPhone 4s is misbehaving. Lets put it in its place and make sure that it doesn't continue to act up going forward.
Cloudmaker provided us with some great steps by forcing a reset by holding down the power button and the home key. Were you able to try that? Is everything up and running?
If this doesn't work we may need to try a hard rest on the device. For steps on how to perform a reset, please visit http://vz.to/uBfksa.
Let us know if you need additional assistance.
Follow us on Twitter @VZWSupport
There are two likely causes of this issue. It's either software or hardware.
I would recommend connecting your iPhone to iTunes and doing a factory reset if this is a software issue, it should fix it.
If this is a hardware issue, it's likely caused by the mainboard of the phone failing, there is no fix for this other than replacing the main board or getting a different phone. If you are under warranty you can make an appointment at an Apple Store and they can exchange the phone for you with another identicle refurbished model. If you are out of warranty you can contact Apple and see what their support options are.