This article explains how to get the Homebrew Channel back after updating your Wii's operating system.
How to Undo a Wii Homebrew and Start Over!
The most common way to get an accidental upgrade is to play a game that has an update checker and you haven't disabled the Wii's update checker. You'll be notified when a new Wii update is available from Nintendo, but you can decline to download it. If you don't decline, your Wii will be updated to the current version and your Homebrew Channel will disappear.
First, you need to know what version of the operating system you have upgraded to. The last update for the Wii was 4.3. To find out what version of the operating system you have, go to Wii Options > Wii Settings and check the number in the upper right corner of that screen. That is the current version of the operating system.
Now you need to reinstall the Homebrew Channel for the correct operating system. Read the Homebrew Channel installation guide to learn how to determine which Homebrew package you need and how to install it on your system. The instructions below work with OS 4.3.