check to make sure the password hasn't expired or changed for the account running the actual service, and/or for the account being used to provide security permissions for users. That is, is you are using active directory and have to change passwords ever 60-90 days, make sure to update the account password that is used to connect to the network active directory (in ArcGIS Server manager).
if possible, it helps to use a service account with a really complicated password (to make the IT folks happy) that never expires....or at least has a long password shelf life.
Also, never hurts to reboot of course as a first try....sometimes that fixes things that just start happening for no other known reason.