Did something eat the tops off, you can continue to sow marigolds in trays into April, from then on you can sow direct into your flower beds. Again the seedlings need to be protected from slugs, snails and birds.

Hi there, I would do more, also do a small tray in a few weeks time, as a back up, if you don't need them for your garden you can always gift them to family and friends. It sounds as if somthing munched your last lot, were they covered? A warm window sill or heated propagator should have them germinating nin no time.
Happy sewing.

I find slugs and snails LOVE marigolds, even big ones have disappeared overnight when planted out. They'd be my first suspects.

I don't even start mine in the greenhouse until end of march or they are too leggy as we get frosts here into may-- also a point, yesterday I found baby snails on a clay pot in the greenhouse.....