Normally to reset your client's view of a TFS server from Offline to Online, you would 'Go Online' with a solution bound to that server.  The server's offline state is reset as part of this operation.  If you want to manually mark your server as Online (or Offline), you can modify the Offline and AuoReconnect registry key values mentioned in: http://blogs.msdn.com/benryan/archive/2007/12/12/when-and-how-does-my-solution-go-offline.aspx.   I

f you want to avoid directly modifying the registry, there is now a UI available to update the values.  This UI is the 'tfpt tweakui' command found in the Visual Studio Team System 2008 Team Foundation Server Power Tools (available at: http://msdn.microsoft.com/en-us/tfs2008/bb980963.aspx). 

To use tweakui to modify your server's offline state simply select your server in the list view, click the Edit button and set the states of the "Auto reconnect this server on startup" and "Server is offline" checkboxes.

Manually taking your server offline could be useful if you are pulling your client machine off the network and want to work offline without waiting for a solution to discover its offline state by timing out its server connection attempt.  Manually taking your server online has the marginal benefit of having your next attempt to open a solution prompt you during the open to take the solution online as opposed to having to manually choosing the Go Online menu option or toolbar button after the solution has opened.