This question in the DSi FAQ (click here) offers this advice.

Question: My Nintendo DSi starts up and it says 0000fe00, what does it mean and how can I fix it?
Answer:  Some users have been able to solve this problem by resetting the  system. To do so, hold down every button while the system is on except  the D-Pad and power buttons. Otherwise if the problem continues you'll  likely need to send in your system for repair. To setup a repair, call  Nintendo's Consumer Service Department at 1-800-255-3700 from the hours  of 6am - 7pm, Pacific Time, 7 days a week.

Try it and see if it works for you.

The DSi comes with a one year warranty that can be extended by 90 days if you register it with Club Nintendo.  If your warranty is up, then a repair will cost around $75.  To schedule a repair and get a definitive price, call Nintendo at 1800-255-3700 (6am-7pm, Pacific Time, 7 days a week).

Please create new threads for new questions.  Also, this thread is well over a year old, so please don't bump old threads.