[karooba007]

onion top\leaves collapsed and gone soft and slimy. whats the cause?
is it a fungus attack?

[Bozwell]

Sorry to read about your onions. I think your problem might be onion leaf blight. Try searching this on the net and have a look for your self there may be photo's you can compare yours with. 

[sunshineband]

There is some advice about onion problems on the info pages here:

http://www.allotment-garden.org/vegetable/onion-problem-disease.php

Hope that helps