1. Photograph a white object under the actual lighting conditions
2. Select [Custom White Balance] under the Camera-II tab
3. Select the photo of the reference object, and confirm selection
4. Change White Balance mode (using the WB button) to 'Custom'

The 550D seems to take a somewhat idiosyncratic approach to dealing with potentially faulty SD cards, and SD cards that have been formatted in a way other than how it expects — rather than reporting a problem with the card, the display fails to light (denying access to any on-screen information or menu options) and the shutter refuses to engage, even though the exposure and focus mechanisms continue to respond.

This has happened twice now, and in both cases, the solution was not just to replace the SD card, but to reformat it too, either in a different model of Canon camera, or under a Windows OS. (And in one of those cases, after 'booting' correctly, the 550D still required a 'local' reformatting before it would work properly.) 0_o