Where have my safari, App Store, iTunes Store, and Siri gone?
Sounds like your restrictions were turned on somehow. Try the following:


Open Settings
Tap General
Scroll down to Restrictions and see if it says it's On
If it is, when you tap it, it will ask for a "Restrictions Passcode". If you don't remember setting this then just try your unlock code or SIM PIN.
You'll probably notice that the toggles for Safari and the others are set to off. If you don't want these restrictions in place then just tap "Disable Restrictions" at the top of that view.
You'll be asked to enter that passcode one last time to disable the restrictions


And that's it! You should now be able to access everything again!

If you can't, there's another possibility that the device is being managed with a provisioning profile so if the above doesn't work, leave a comment and let me know. :)