is it possible to login on a remote machine when the ssh service has been stopped?

"X windows is running
how do i use x server to log on to that machine.

There's only a very small chance you can. Just running X is not enough; it also needs to have been configured to allow remote login (such as XDMCP). And any firewall or router needs to have the ports configured well (TCP 6000-6005 for X11, UDP 177 for XDMCP, and optionally TCP 7100 for the xfs font server), for otherwise it will only work using ssh -Y after all. A sane administrator would probably only allow access through SSH anyway.

But, you could try Xephyr (or the older Xnest, or XWin.exe in Cygwin), using something like:

Xephyr :1 -query name-or-ip-of-remote-machine

XWin.exe -query name-or-ip-of-remote-machine

As you actually do have physical access, you might be able to set up the machine to your needs, but I assume setting up some VCN to some existing GNOME or KDE session is easier. Or set up something like monit to auto restart SSH when it dies. (See Monit: Monitor SSHD Server and Auto Restart SSH If It Does Not Respond, and to install use something like sudo apt-get install monit if applicable.)"

"Yes, If telnet is there, its possible to login to that box using it even when ssh is not available. Try using putty to connect using telnet. The default port for telent is 23. If you use putty, it will show default ports whenever you select an option.

If telnet is not there, you can see if rlogin is there. The default port to connect to is 513. The remote machine should be running a rlogind daemon for this to work."