If your iPhone's screen becomes unresponsive or "frozen," your best bet is to power the phone off and on again. If this doesn't work, try to reset your phone by holding the home key and the sleep/wake button at the same time. If it's still frozen, you can connect your phone to your computer and use iTunes to restore the OS. Remember, you should always back up your data before restoring in iTunes.
This can be a tricky one. Try connecting your iPhone to a power supply and charging the battery. The low battery screen may take a while to appear, so be patient. If that doesn't help, it's possible that you may need to contact Apple's service center or get a new battery.
If your iPhone won't turn off try forcing it off by holding both the sleep/wake button and the home key for about five seconds. Continue to hold until the Apple logo appears. Your phone should now power on and off normally.
First, make sure WiFi is enabled in your phone's settings menu. If it's still not working, there are a few things you can try. First, power your iPhone off and on. If that doesn't work, try to restore your device's settings by holding the home key with the sleep/wake key. If your iPhone is having trouble connecting to one specific network, try to force the phone to forget that network. (Settings » touch network name » choose "forget" on next screen). Power off and on, then try to reconnect.
It's possible that you have restrictions turned on, so first go to settings » general » restrictions and make sure the "allow camera" option is set to on. It is also a good idea to tap "disable restrictions." If this doesn't work, try powering the device off and on or try to reset the device (sleep/wake key with home key). As a last resort, back up your data and then try to restore your phone in iTunes. If none of this works, it's possible that there's an actual hardware problem with your camera, in which case your best bet would be to contact Apple or take the phone to your nearest Apple Store.
If you own an iPhone 4S or iPhone 5, Siri is probably your new best friend. To get her working properly again, try cleaning the microphone on the bottom of the device first. Next try to toggle Siri on and off by going to settings>general>Siri turn Siri off wait 30 seconds, turn back on. If you still have no luck try to reset the network as Siri runs off your data. Reset the network settings here, settings>general>reset> reset network settings. Last but not least you may have to reset your entire device. Go to settings>general>reset>reset all settings. This will not wipe any data fro your device and will reset all setting to factory.