Hello Filiberto,
you haven't provided enough information to allow for help.
Try to add a network diagram, the type of WAN connectivity and the network devices in use.
>>
why a few user can access without problem.
if the users still working are in the same location(s) as the other ones this might be a problem at an upper ISO OSI level then level 3.
there is a FW on the path?
if all the still working users are in some locations and the other not working are in different locations it might be a network problem.
It is not possible to say more without further details
Hope to help
Giuseppe