Can you remember what VST you were adding?
Sounds like a 'bad' VST made XT2 disappear.
I know that other hosts handle VSTs better in a way so that if they crash the host itself won't crash too.
If XT2 crashes often/reproducible with that VST then you should let Jorgen (XT2 developer) or the dev/s of the VST know it.
I sometimes have problems when changing VST presets, loading insert FX, or adding VSTs while the sequencer is running. I try to remember to stop the sequencer when performing these actions, especially if I haven't saved for a while.