Iphone 4s volume bar has disappeared

My daughter had the same problem.  Her's totally disappeared.  I found where someone told me to try putting in the earbuds and making sure they worked fine.  Then I plugged them in and removed them a couple of times and the problem was fixed.  She has not had any problems since.  Hope this works for you.

I have had the same issue before and have found that it can be the headphone jack, as sw3019 suggest, or the dock connector. Plugging in the headphones may help (note: you may have to do this several times), as Apple has even suggested this in a tip sheet

http://support.apple.com/kb/TS1205.

Also cleaning the dock connector with a very soft pipe cleaner or something similar. If dirt gets in the dock connector it may think the dock is connected to a source and not play out of the speaker, only the headphones. I know this can be very annoying.

Hope this helps.