Several factors can cause a laptop to be unable to connect to a wireless network, such as low signal strength, a faulty wireless card, or not having the wireless security password. However, an inadequate signal is one of the most common reasons for not being able to connect.
Low signal strength can happen because of atmospheric attenuation: that is, the natural decay of waves as they travel through air, or interference from electronic devices such as cellphones or even other wireless networks. Wireless network routers commonly transmit at 2.4 GHz, which is the same frequency as cellphones and microwaves. If any of these devices are in the way they interfere with data by producing electromagnetic noise. Also, not being close enough to the network router can cause lack of signal. By opening the list of wireless networks in the System Tray, it's possible to find out how strong a network signal is by looking at the bars. One or two bars is a weak signal and less likely to connect.
Inability to connect can also indicate a problem with the laptop's wireless network card or driver. Checking Device Manager and reinstalling drivers may fix the problem. If not, a new network adapter or USB stick can help. Finally, if the network is secured, the user needs the password to connect and get past the encryption.