It could be one of the plugins causin it, can u access plus at all?
If so disable every plugin (plus > preferences > plugins... uncheck all the boxes infront of the plugins) and restart msn messenger..does it start up ok?
Then check one plugin at a time an restart msn again..do so until u find wich plugin is causin it and delete that one
If u cant access plus at all i suggest u uninstall (including all settings and plugins) then reinstall (u can install plugins again fresh...but do it one by one..an test if it still goes alrite).