A few questions:
Which USB loader are you using? WiiFlow, CFG, GX
What format is your USB drive? WBFS, FAT32, NTFS
I'm assuming you now have the d2x v8 cIOSs or did you change it in Modmii?
Do you know what version you had when the USB drive worked?
I had an issue with games not showing up once, I'd just transferred games from my old USB drive to my new USB drive using a program on my computer as a MITM. On the new FAT32 drive, my wbfs folder was named in all caps and my loader didn't like it, had to make it lowercase.
If none of the loaders can see your drive/games maybe try dx2 v9 or v10. You have to run Modmii classic, white text on blue background, goto download page 4 type beta then change to v9 or v10 and get 24956 and 25057 when back on DL pg 4. Install .wad files with MMM just like before. (You should remove all the .wad files on your SD card from when you did the guide then put just these 2 in that folder and install them)