Please answer the following questions so we can get a better understanding of what you are seeing:
- Have you ensured your router is broadcasting a 2.4GHz Wi-Fi network for the device to connect to? The Roku Express/Premiere is only compatible with wireless b/g/n networks on the 2.4 GHz spectrum. - Have you tried changing the wireless broadcast channel on the router to see if this will allow your device to connect? - Have you tried moving the device closer to your router to see if you can get connected? - How long have you been experiencing this problem?
If those things still aren't helping, feel free to provide us with the serial number of your Roku from Settings>System>About along with the answers to the above questions and I'll be able to assist you further from there.