Itunes will not open

My OS is a Windows 7 Home premium.I used to have no problem running itunes 11.3.1.My internet service was shut down temporarily and while i was down, i tried to open up itunes and it kept saying that i had a missing dll and to reinstall itunes.When i got my net back on, i tried reinstalling itunes but now it won't even open up.I have allowed it to run through the firewall and other virus software and my pop up blocker is off.Wh