Dropped ssh/X session, how to pickup where left off?

You cannot reconnect to a session you already lost. You can only kill its processes.

But if you are preparing for future, you can use tmux or screen for terminal-based programs and xpra (this fork) for X11 ones.

tmux
tmux ls
tmux attach

xpra start :100 --start=xterm
xpra attach :100