the same thing happened to me yesterday, but in the month view (ironically my first choice of view). somehow the com.apple.iCal.plist has become corrupted and needs to be rebuilt. the file is in your preferences folder, and needs to be removed, causeing iCal to rebuild the file next time it launches.
first quit iCal, remove the file, then restart iCal.
you dont have to delete the file to start with if your not sure, just move it out of the directory (say the desktop) and then when you see the program working just fine with the new plist, delete the old one.
the preferences folder is in your home folder's library.