Hello,
The problem is caused by the code you added in custom.css file:
.tabs {
display: none;
}
You removed all tabs that’s why accordion tabs on checkout can’t be displayed.
If you want to hide woocommerce tabs you can refer to this documentation:
http://docs.woothemes.com/document/editing-product-data-tabs/
Thank you.
Regards,
Eva Kemp.