Chimney cap flew off

If its metalbestos or duratech they had twist on caps. If its an air cooled or dense packed class "A" chimeny 3 sheet metal screws are typicall sufficient to mount it. If its a clay flue tile get a hammer drill and a masonry bit slightly larger than the bolts. Put the cap on the tile and tighten the bolts so they make a score mark in the flue tile. Remove the cap drill holes about 1/4" to 3/8" deep in the flue tile. Don't go through the flue tile.

Some silicone caulk will help with most caps.

It should have the bolts in the cap? Are there threaded bolt holes on the base of the cap? I can't remember the standard diameter of bolt, the heads are 7/16" though. Does your cap have claw tabs that hold the cap on by tension? I used to run into those alot. Can you post a picture of your cap?

Also what Craig said about the silicone works well, just don't glob it on, remember you have to take it back off to clean it!

Without know the exact cap, it's tough to give good advice! If you don't have any bolts, then it is probably one of those with an expanding inside that hugs the flue interior. That should be bendable or adjustable for a tighter fit. In any case, add some silicone on the flue tile top edge.