Hi,
it is a sudden case or it happens after some configuration changes in your infrastructure?
How they appear in the inventory HC view as disconnected (grayed), with red-cross sign...??
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?
There are too many unknowns so please be more specific...
Please post vmkernel and fdm.log from hosts.
Anyway as an reference for hosts diagnosis see: