You can follow the article: Creative Cloud applications ask for serial number and perform the steps mentioned in order to get your issue resolved.
Let me know if it works or not.
Adding to previous resolution, just wanted to mention that do check the host file too as they block Adobe at times.
Creative cloud do not need a serial number.
In case it is not signing in successfully please try the following:
I don't know which operating system you are working on so i am giving you some steps windows and MAC OS:
In windows 7 navigate to following location:
1. look out for "Hosts" file
2. Open it with notepad
3. Check if you have any entry for Adobe
4. Remove the entries and try again launching any product from CC
1. Please click on "Go" and navigate to /private/etc
2. Open "hosts" file and check out for any entries for Adobe.com
3. Remove the entries and save the file
4. try again launching any product from CC