If your Wii U no longer boots, follow these steps:

1. Power off the console and remove the SD card
2. Power the console back on

If you installed stroopwafel to the SLC (Internal Memory):

1. Power off the console
2. Press Power, wait a second, hold Eject for 5 seconds

Now the console should be booted stock. Launch Wafel Installer again, and use the Stroopwafel plugin manager to remove any problematic plugins.

For further assistance, please visit the ISFShax thread on GBATemp.