unblock a blocked device

You have to log into your router and navigate to the wireless section.  The block should be listed in a permission section.

Unless you mean the firewall on your computer warned you of the "suspicious computer" via a pop-up message, and you clicked OK to block it.  If so, you'll need to open that firewall and unblock the IP address.  Just delete the entry and the next time it pops up, allow the IP address.

If you have a Panoramic modem then blocked devices are listed under "Parental Control > Managed Devices. Click on the X to the right of the blocked device to unblock it.