Hello. Try downloading and installing a fresh copy of your Conexant audio driver. Go to the Inspiron Mini Duo 1090 support page . Expand the Audio category and obtain the 1st driver (not the dock driver). Note that it is only for Windows 7 32-bit.

If there is a configuration issue or if the driver files had become corrupted, the fresh driver will fix it.

Can anybody explained why the mic stopped working

The mic could have just failed as all things eventually do. To diagnose hardware failure, reset the laptop back to the original factory configuration (perform a system recovery). Either the problem will go away (if it is a software issue) or it won't be fixed and will be confirmed to be failed hardware.

Thank you. Did all you said but mic problem persists. Sadly i think i have a failed hardware problem. Any tip on what to do?

You could use an external mic plugged into the mic jack socket, or a usb mic plugged into a usb port.

To confirm hardware failure of your built-in mic, reset the laptop back to the original factory configuration (perform a system recovery). Either the problem will go away (if it is a software issue) or it won't be fixed and will be confirmed to be failed hardware.