I had one do the same thing and I could never get anything that would reattach and hold it. I even contacted an inventor who works for Loctite. He sent me the best stuff that they make and it didn't hold.
You state keel guard. Is it a keel guard ,keel shield, or Hamby's. If you know or can find out then call the company. Some of them have a full replacement. Others give you money off when buying a new one.
Mine is a Keel Shield and they gave me $100 off on buying a new one with lifetime guarantee. So far it is holding.
I don't know what kind it is. I have not seen a name on it anywhere. I doubt it is a Hamby. Don't they have their name on theirs?
How do I find out? Also what did you use to get the old adhesive off the boat?
I can't tell you how to find out what it is. Look at on line pics and yours you may be able to tell.
What I did is rip mine off and take a bunch of acetone. I put it in a spray bottle and kept the area wet a few inches at a time. I used a plastic putty knife and scraped off the adhesive as it disolved. When I got down to the final residue, I sprayed it and used a scott pad since the area has to be roughed up anyway.
It was slow and laborious. Also you need to have plenty of ventilation with acetone.
Now all you have left is the work of taking the old one off. Be patient when trying to get the old glue off. Be sure to clean the area well and scuff it up with a scotch brite pad so the new one will stick down well.