You could reboot the server that WSUS is running on, but if it's used for other things also (e-mail, dhcp, dns, file, printing, etc...), you'll have to wait till there's downtime, before trying again.
Another option though is to restart the database service.
If you go to administrative tools -> services (or runbox services.msc), look for 'Windows Internal Database" and right click on it, and select restart
or from an admin command prompt:
sc stop mssql$microsoft##wid
sc start mssql$microsoft##wid
Then go back to the wsus management console, click on reset server node, and you should now be able to run the cleaning wizard (it'll probably take a couple hours or more, but it shouldn't give you an error),
And you were able to do this without interrupting whatever else may be running on it.
