The following links contain information for maintenance and resetting of stuck and unresponsive windows.
They mainly refer to windows with the chain connected and operated by the remote control.
Unsticking usually requires an appropriately sized paper clip and knowledge of the location of the chain and reset button.
It is sometimes possible to get a window working again by removing from the remote then re-adding. If this doesn't work it needs to be reset (little white button) then removed from remote and re-added.
Where the chain is not connected and they are being operated manually there is usually no problem.