If you are more specific with your problem it will help us to answer the question correctly. Below are few question which if answered may help out.

When did you reflash your 770
How did you flash your 770
What image did you flash your 770 with
How many repositories are listed
Which repositories are listed
Was there a program that you installed and then it stopped
How are you connecting to the internet
are you beind a proxy
What does the log file say
Is there any error that comes up
Are there any other programs running
etc...............