Yeah, updating wipes out Priiloader. Messing with themes without fully understanding what you're doing is a bad idea, as you've sadly learnt the hard way. For the record, Multi Mod Manager is not good app, it is a poorly coded rip off of a bunch of other programs, with numerous bugs added and without releasing source (some or all of the apps it has stolen code from are GPL licensed, which basically means that when code from them is used, the source code must be released).
Sadly at this point there is not much that can be done to fix your Wii. There are a few questions that if you could answer, it would give us an indication if there's anything you (or someone else) can do to fix it.
1. Do you have BootMii/boot2 installed? (If you don't know, you probably don't; BootMii/boot2 is a piece of software that can be installed via the HackMii Installer when you are installing HBC, but it can't be installed on newer Wiis. Do not confuse it with BootMii/IOS, which can be installed on all Wiis)
2. Do you have BootMii/IOS installed
and a NAND dump made using it? (Again, if you don't know, you probably don't; BootMii/IOS is automatically installed when you run the HackMii Installer to install HBC, but unless you actually run it, it will not make a NAND dump. If you do have a dump, it will be named nand.bin - you can't download this file from elsewhere, it is specific to your Wii)
3. Do you have a modchip and/or a patched System Menu IOS (e.g. an IOS from cIOScorp/Darkcorp)?