OKAY, I know this sounds silly - but before I would do ANYTHING I would from my personal experience with strange things on iPhones do the following part A & B.
(A)
- Turn on assistive touch.
- Tap the white circle.
- In rapid consecutive steps press but do not hold: volume up, then volume down, then the sleep/power button.
- When the slider appears so step 5 below.
- Hold the home button until the iPhone reacts.
- Enter your password.
- After back to home screen then tap the white assistive touch home button from your screen then tap restart.
If Part A alone does not resolve then follow step B while repeating step A first.
(B)
1) Airplane mode on & then off, waiting 30 seconds between swipes.
2) Any time you delete data like pics or update large number (or any) apps, do 1 and then restart.
3) If #2 was not necessary, restart phone anyway - I like doing It through assistive touch which can be added to your control panel... or shut down through General.
4) Every day / week at least once if none of 1-3 were necessary - power off the phone for a minute and power back on.