Hello andreat84 and welcome to the ETS forums!
Sometimes it does happen that the empty server is set to a custom map after people left it that way. If this prevents you from playing feel free to use the shoutbox on our website to call for an admin who can reset the map remotely via webadmin interface, or, for the future I guess it'd be easiest to follow its0's link and download said custom maps. They're fun!
Unfortunately I do not know of a mechanism that will reset the map to a vanilla one once the server is empty but I'm sure Hunter knows more about that. We are sorry for any inconvenience.