it is a sudden case or it happens after some configuration changes in your infrastructure?
It is a sudden case
How they appear in the inventory HC view as disconnected (grayed), with red-cross sign...??
There are in normal state
Also verify your account permissions on hosts I suppose they are not joined to the domain so your direct access account differs from account you used for login to vCenter server. I am I right?
I add host to vcenter using root account and my host joined to domain.