If the old Canon SLR is also an "EOS" model (easy to see at the label) then the answer is a straight "yes". No camera anymore? OK, take off the lens cap from front, look at the lens from front. There are some details about the lens printed there, quite similar to your current lens on the 550D. If the old lens has "Canon [Zoom Lens] EF xxx" printed there (most important is the "EF") then you can use this lens on your 550D.
Canon EF is the lens mount system for all Canon Auto focus cameras and Auto focus lenses.
Whether this lens is of any use (due to age, image quality and maybe fungus) is a different story. Best to post a picture here.
The question presented is not stupid at all but the manner it was presented was not very good. Do present your question with as much info as possible. People here are extremely helpful but they are are not mind readers. Details such as specs and description of the lens was completely left out. If words can't describe, then picture paints a thousand words.
Hope to get more details. I'm sure alot of us are quite curious.