Cant connect to internet

Hello,
my airport connection is strong but i cannot access the internet. this is a new problem. i live in an apartment and pick up the signal from the coffee shop next door. no password is required. like i said, the signal is strong. when i try to open the internet it searches for a while and then says it cannot find the server. i have been able to get on for the last 4 months and suddenly 4 days ago this problem arose. please help if you can