Alright, i'll be a little more curteous.
What do you mean "nothing happens?" Does the Steam window not show up? I'm assuming this is true. If it is, you need to shut off any and all firewalls and/or spy/mal/adware and virus blockers and scanners. You may need to actually uninstall said programs. Have you installed any new programs between the time you were last using Steam and the first time the error occured? If so, try uninstalling that program.