I came across this while researching another Celebrity issue I'm having...
My first Celebrity cruise was in 1997 and I did not receive points. I contacted them about 2014(!) when I next cruised Celebrity and they wanted proof... I emailed them a photo of us (one of the $$ one that Celebrity takes with us next to the Galaxy character-I don't know what he was but he had a Galaxy outfit on). That was good enough for them and they gave us points.
So if you can find a photo that "proves" you were on the ship, that may help.