Did you tried resetting preferences? Check the link on how to do it http://helpx.adobe.com/fireworks/kb/restore-preferences-fireworks-cs5.html
What Operating System do you get the issue on?
Do you remember making any H/w or S/w changes on the computer having issues?
Are you using a mouse or a Pen Tablet? Did you tried updating their drivers?
Make sure you did not locked any of the layer mistakenly.