I received a call from a client this morning that Skype wasn't connecting on her computer. After a quick test we found it was working now, but I think it was being caused by being on the fringe of her wireless network.
Wireless networks can be effected by building materials and radio interference from other devices and you may find that you have "dead spots" in your home or office where the wireless signal is too weak to be reliable. Try moving to a lightly different location and see if the problem solves itself.
Of course, services like Skype can also have their own outages and maintenance periods, so the problem could be completely outside your control. Skype, and other good services, usually provide some sort of status page where you can find information on outages, maintenance windows and other issues. For Skype, you can find this info at http://heartbeat.skype.com/