Sorry about your wii. Unfortunately I don't have any good news for you, but I can try to explain why your wii is bricked, at least what I likely think happened.
The most likely cause of your brick is a failure in the priiloader installation. When you install priiloader it replaces the wii system menu dol with priiloader and moves the system menu to another file. That is why when the wii turns on you can get priiloader before the system menu. This is a well tested hack, however if it fails to install it is very bad. Unfortunately, very rarely priiloader does fail to install, some installer versions are known to fail more that others. So in your case, just very bad luck. Installing priiloader is highly recommended, and will likely continue to be so. The important question for the community to know was which version did you install? Priiloader 0.7, priiloader 0.7 236 mod, or priiloader beta 0.8 v?, or other. (my guess is beta 0.8, but I could be wrong)
It is also possible, but less likely that your downgrade failed, either the system menu install or the cios60 install. Not very likely at all, if you where paying attention and every wad installed then there should be no issue.
Sorry, my money is on priiloader. Unfortunately not the first time or the last, but it is rather rare.
The only way to fix the wii is the send it with your nand keys to some who programs wii nands, like deadlyfoez, krafter, streamlinehd, all in the usa. Or learn to do it yourself if that is your thing (not mine).