Please can you tell me why my Cucumbers collapse after they are 3/4" high, They are on a cool window in fiber pots as later I want to put them in the greenhouse
The phrase 'as cool as a cucumber' doesn't really make a lot of sense. They require lots of heat, sunshine, to grow: a cool window is not a good place for them.
It is a little early to be starting cucumbers. As Stewey has pointed out, they need good heat and light. It is also possible you are overwatering them and the seedlings are damping off.