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The mirror may have fallen in yours. Here's a link for how to repair that problem. If it is electrical, then maybe check the connector and possibly the HUD went bad. You can get one off eBay.
I just went through this, after my display itself (not the mirror) failed, looking for a new (used) HUD display, and the prices asked on there and from the vendors on here, range from $150-300, and all of these are for used units taken out of another C5. I seriously doubt you'll ever find a new one.
And if you don't disassemble the dash yourself to get it out and replace or repair it, be prepared, in addition, for a hefty bill from the stealership.
If you can find it's because of a falling mirror and you fix it yourself, in addition to your time, the cost will probably be less than $1.