Q: I am unable to connect to wifi using my phone since yesterday night , please advice

Hello Smitha4400,

It sounds like you are unable to connect to a Wi-Fi network on your phone. I have an article that I recommend the troubleshooting from named:

iOS: Troubleshooting Wi-Fi networks and connections

http://support.apple.com/kb/ts1398

1. Be sure you're in range of your Wi-Fi router (access point).
2. Tap Settings > Wi-Fi and turn Wi-Fi off and on. If your Wi-Fi setting is dimmed, follow these steps.
3. Confirm that your Wi-Fi router and cable or DSL modem are connected to power, turned on, and connected to the Internet. If not, refer to your network administrator or Internet service provider (ISP) for assistance.
4. Restart your iOS device.
5. Tap Settings > Wi-Fi and locate the Wi-Fi network to which you're connected.
6. Tap and Forget this Network.
7. Try to connect to your desired Wi-Fi network.
   Note: You may need to enter your Wi-Fi password again if your network requires one.
8. Turn your Wi-Fi router off and on2. If your ISP also provides cable or phone service, check with them before attempting this step to avoid interruption of service.
9. Update your device to the latest version of software.
10. Update your Wi-Fi router to the latest firmware2. For AirPort Base Stations, install updates using the AirPort Utility.

And this section from down the bottom if needed depending on what the phone is doing:

If the issue is still unresolved

Follow the steps below for the issue you're experiencing.

Collapse All | Expand All

Unable to locate a Wi-Fi network

Unable to connect to a Wi-Fi network

Wi-Fi disconnects or signal strength is less than expected

Unable to access the Internet while connected to Wi-Fi

Unable to automatically connect to Wi-Fi

Thank you for using Apple Support Communities.

All the very best,

Sterling