I always recommend starting at the bottom, the physical layer. Is the device on? People accidentally bump the on/off switch. Is there any RF noise causing a problem? There are a lot of non 802.11 sources of layer one problems. If the air is "clean", I then look at layer two. Can the device connect? Often people are not looking for the right SSID or they are using the wrong credentials. If the device can not authenticate, it can not associate. Layer two association is required to contend for the medium and make it to layer three. If they are connected, did they get an IP address from the correct scope? With the explosion of BYOD, many IP scopes run out of addresses. If they got an incorrect IP address, are they on the right VLAN? Many layer three and up problems get blamed on the Wi-Fi because that is how the user is connecting. Of course user error can not be overlooked. They may be on the guest network vs. the internal network too. There are several variants to both your question and the answer.