Hi N.Timea

I suggest that you check under device manager if the driver for the network adapter is installed.

Click on ‘Start’ and in the Search box type ‘Device manager’. Click on ‘Device Manager’ folder. In the ‘Device Manager‘ window, click on the triangle to the left of ‘Network adapter’. In the drop down list check what is mentioned, and if there is yellow bang next to the network card, the driver has to be reinstalled again.
Click on the below link to install the driver:

http://www.dell.com/support/drivers/us/en/19/Product/inspiron-15r-n5110

Once the above is opened, click ‘Network’ and then click on ‘Download File’ for ‘Realtek-Driver. Follow the onscreen instruction to install the driver.

In case nothing is showing up under ‘Network Adapters’, click on ‘Other devices’. Under ‘Other devices’ if ‘Ethernet Controller is showing up, then there is no driver available for the system. In that case, please install the driver from the same link above.