Probably something messed up the program preferences - it hapens rarely an is file/last activity specific -...(hapens also on commercial version)
Solution:
You did not say what is the paltform You use - I am usin a PC so I can dirct You to PC solution:
Go to "DRIVE":\Documents and Settings\"USER NAME"\Application Data\Graphisoft\ArchiCAD 9.0.0 INT R1
and delete or rename file "ArchiCAD 9 Basic.prf" (If renaming rename the extesion - the part after ".")
On next start Archicad will rebuild the preferences.
Best Regards,
Piotr