Lisamim
If you are seeing other wireless networks available, but not your own, it may just be a network setting. (You didn't mention the model numbers of your other Roku devices so cannot determine what network band they are capable of using). Should be at least seeing the 2.4ghz band if you are using a wireless channel in your router settings between 1 and 11, and using a Wireless Mode/Protocol of b/g/n.
If your 3820 is seeing no wireless networks at all, (yours or neighboring networks), then good possibility that the "long range wireless receiver" in the power cable may have failed. This receiver is required for the Streaming Sticks 3810 and 3820 models and cannot find or connect to a network if not working properly.
If your issue is the first, try using a USB power adapter at the wall instead of using your TV's power port if currently doing so (to rule out underpowered device), and perform both a System Restart of the device and a Router reboot. Also try establishing a new connection through Settings/System/Advanced System Settings/Network Connection Reset/Reset Connection.
If your issue is the second one (no networks at all), you will likely need to contact Roku for a replacement power cable with the built-in Long Range Wireless Receiver (WR002) from Roku. I believe they are still issuing them as part of warranty since I do not see an accessory listing for the WR002 on the Roku website. I only see the WR001 which is compatible for the 3810/3811 models, not the 3820/3821 models.
Of course, also try the power adapter at the wall like recommended for the first issue, again just to rule out a power issue with the TV USB power port that may be adversely affecting the receiver.
