There are a few ways to diagnose and address this.
1. Assuming the WIFI adapter is detected by Ubuntu and ready to operate.
See if the WIFI is disabled via hardware. For instance many laptops or other systems have a soft button or special function key sequence to completely disable the WIFI; so make sure that you don't have it disabled. In the upper right of your desktop there should be an icon in the menubar, several actually; one of them should be for the WIFI network. If you left click on that icon and see a menu with things like "enable" "connect" and so forth, then firstly make sure it is enabled and then try to connect to your intended WIFI network. If you have a passphrase, it will prompt you for that.
If there's no icon or if there is one but it doesn't seem to show that there's a WIFI device, then it may be disabled by hardware or may not be detected, you'll then have to diagnose further.
2. Determining whether or not the WIFI adapter is detected in your system.
Is it a USB plug in device or internal to the system? Knowing if it is USB is helpful because you can list all USB devices detected in your system via a command.
Open a command prompt, also known as a terminal so you can type commands. Try typing
so you can see the wireless devices detected in your system. If you do not see any devices with names like "wlan0", or "wlan1" then there's a problem detecting your WIFI device.
If your WIFI device is not detected, you can try some things to get it working, at this point it becomes a bit more advanced. A recommendation is to ensure that this WIFI device operates in another operating system, such as Windows and to also find out what exact WIFI device it is. Hopefully the first things work and you're able to get running.
If there is a WIFI device detected, then there are commands you can enter to get it running via the command prompt/terminal window. However it should have been accessible from the window manager.