Two likely possibilities
1- You installed a bad channel wad and have "bannerbricked" your wii. This is fixable by uninstalling the bad wad. You must be careful that the wad is a channel wad, don't uninstall any ios or your system menu. This is simple if you have boo2 bootmii or priiloader installed, if you do not it becomes more difficult. If you do not have priiloader or boot2 bootmii installed, the only way to fix the issue is via savemiifrii. There are many possible methods with savemiifrii, updating the SM will not fix the issue. You must install a modchip of have a darkcorp/cioscorp installed to load games. If may be possible to load a recovery disc from RM is you patched your system ios, if not you may be able to use the disc swap method to load a patched ios and run a recovery app. There is also the wiifit disc trick and the Nand fill trick sddm. These options are detailed in this thread in post #1 and post #157
http://gbatemp.net/t194182-new-unbricking-method-for-bannerbricks-4-1v-wii-s-and-below?&st=157
2- If you did not install any wads and recieved this error by your own bad luck, you may have a corrupt message board. You can test this by loading the wii into maintenace mode. To do this you must hold + - at the health screen, then press A while holding + -. If the system menu loads into maintenace mode you most likely have a 'Mail Brick". To fix a mail brick you must go to the wii settings and do a system format, which will fix the issue and also remove all your saves and channels. You can also fix it without formating with the following method-
download Det1re's Mailbox Brick Fix.
http://det1re.de/d/hb/mailbox_brick_fix-det1re.rar
1. Copy "apps" and "fstoolbox" to your SD card root and overwrite if asked.
2. Startup the Wii and boot HBC (you can use BootMii, Preloader or the Maintainance mode)
to do so.
3. From HBC, run FSToolbox r50 and follow the on-screen instructions until you see your
Wii's file struct (import, meta, shared1, shared2, sys, ticket, title, tmp).
4. Navigate to shared2/wc24/ and replace any file with the one from SD by pressing PLUS.
5. Navigate to shared2/wc24/mbox/ and do the same there.
6. End the app and restart the Wii. You will get a message that your Mail account is broken
and will be restored.
Doing the steps above will remove all your saved friends and all messages from your
Message Board.