Hi Marydoll:
Not sure what exactly you mean by saying "Canon 550D's handgrip has fallen off?
Is this the skin cover of grip that is fallen? Is it attachment from camera body?
No mater what, if it is under warranty period, you are in luck and send it to Canon for free repair.
If it has a major issue and you have no warranty coverage, I would try to make a duck tape fix myself and literary Duck tape has saved me with my own broken door cover part of grip (My fault so not to blame any manufacturer). It is still in use (On my 5D MK II).
If you can't fix it, I would suggest to have a look on very good third party handgrips.
Look at this with
2 set of batteries >>> LINK
I have one of these for my T4i that should be same grip for you and love it unconditionally specially the LCD / timer / time lapse features.
>>>> LINK
You can't beat the price compare to simple original Canons and quality of what I got is very good, though as usual there are mixed reviews. See the reviews (Not only amazon but other photographic sites).
Wish you luck!