An Error99 could be anything, absolutely anything could have gone wrong with it.
Does it always show Error 99? or does it only show Error 99 every now and then? Is it when you do something specific?
Try switching your SD card, battery, and lens and see if that helps
Any EF or EF-S lens should do and any SD card should work
Batteries on the other hand should be genuine Canon battery or a reputable generic brand.
If this does not work, contact Canon and they will tell you the closest repair facility you can send it to to have it checked out.