It seems the Fn key got disabled in Windows Mobility Center. Follow the steps below to enable Fn key in Windows Mobility Center:
1. Click "Start", type "Windows Mobility Center" and press enter.
2. In the Windows Mobility Center window, please locate "Function Key Behavior".
3. Click drop down arrow next to "Function Key Behavior" and then select "Function key"
There is another way to change the Fn Key behavior in Inspiron N4110. Follow the steps below to change the Fn Key behavior:
1. Power on/Restart the computer.
2. Start tapping the F2 key when the Dell Logo appears.
3. Enter the System Setup (BIOS).
4. Go to Advanced tab by pressing the right/left arrow key.
5. Using the down/up arrow keys select Function Key Behavior and hit Enter.
6. Choose Function Key and hit Enter.
This might happen due to a software problem or the problem might be with the actual key itself. I would like to know when you are not able to use this key. Are you using the Fn key to navigate to the BIOS or Run diagnostics or this is happening only in Windows? Also reply with the Dell Laptop Model Number in your post.