There are several reasons why you might be able to see a wireless network but not be able to connect to it.
1) The Access Point (AP) is using MAC filtering, and your MAC address is not in the permission table.
2) The AP is set to allow a certain number of connections (for example, it allows 5 users to connect at any one time), and all connections are in use. This is usually adjusted by controlling the number of IP addresses the AP can assign with DHCP.
3) The AP is configured as the gateway to a VPN port, and your laptop isn’t configured with the right credentials.
4) Signal strength may be inadequate to sustain a connection.
5) The AP may need to be rebooted.
6) The laptop or its wireless card may need to be rebooted.
7) The network behind the AP is currently turned off. If the AP is connected to power, you’ll still see the AP, even though you can’t connect to the network behind it.
You didn’t mention encryption or authentication, so I’m assuming you’re asking about an “open” system. Naturally, it will be difficult to connect if the AP is using WEP or WPA encryption, or is connected to an authentication server, unless you’re an authorized user. I listed the VPN scenario (item 3 above) separately from this paragraph about encryption and authentication because the AP can truly be “wide open” – no special configuration at all – if its only TCP/IP connection is to a VPN. Unless you’re authorized, you won’t be able to connect, but you’ll see the AP just fine.