Skype has a lot of bugs, especially with Vista and Windows 7. Did you know Skype is actually incompatible with these two OS's?
In regards to your initial question, try checking your Internet connection unless you can actually send and receives messages, calls, etc.
If that's the case, click the icon beside your name in the main window and see if it's green. If not, scroll down and see if you can click the green status "Available".
If it is green, and you mean that your contacts see you offline but you're not really offline, that is a bug, and it happens a lot with me (probably because of the incompatability). You can't do anything about it until Skype is developed for W7 & Vista systems.