Certain types of onions store better than others. They should be harvested once the leaves flop over at the neck, cured for about a week, then stored in mesh bags with plenty of air circulation. Here is a video which outlines the steps to successfully harvest & store onions. After watching, you can ascertain why your onions didn't hold up.

How to Harvest and Store Onions
https://www.youtube.com/watch?v=py2kPbSsgmA

Could it be that you didn't allow them to dry out in the sun before storing? Any with thick necks would need to be used first. Did you use the same variety that you normally grow? Otherwise, try a different variety next year and see if you do any better. Personally, I no longer grow onions but I found Turbo did really well for keeping.

its been a very dry summer and I suspect the environmental conditions haven't helped in there growth, development and ripening. Have you grown the same variety that you normally do or a different one?

Thanks for your replies. The variety was same one, but may try Turbo next year. Not tried that one before.