macbook pro cannot connect to wifi

before i let apple fix my laptop cos of the malware, the wifi works fine. then i got my laptop last monday and i tried connecting to the wifi in the apple store and it worked okay. when i got home, i cannot connect to my wifi anymore. well it can detect the wifi but it usually states connection timeout and invalid password (rarely when i keep on connecting to the network even though its the right PW) then when i finally get connected, it'll be like NO INTERNET CONNECTION and this "!" will show up in the middle of the wifi status...

please help.