You posted this in 'OS X - Operating System', but it seems that you're talking about an iOS device. So, which is it?
Perhaps you can give us some specifics about the problem, the affected system (version, age, etc), and the wireless network (router brand, type and what troubleshooting steps, if any, you have already taken).
Posting a question like this is akin to calling a motorcycle shop and saying "I have a problem with my Corolla - what's wrong?". Imagine the response you'd get.