Is the Time Sheet Notifications Service still running? Can check in Workbench via Sys Admin ---> Server Tools ---> Service Controller Report.
Has email server location changed recently? If so, the server.conf file needs to be updated to reflect the changes. If not, have the people who manage the email server take a look to see if any of the mails are stuck there.
Have any of the following config items been changed or "turned off" recently? EMAIL_NOTIFICATION_CHECK_INTERVAL , TURN_ON_NOTIFICATIONS, ENABLE_TIME_SHEET_NOTIFICATIONS_SERVICE, TMG_DATE_NOTIFICATION_INTERVAL
Has the Time Sheet Policy or its notifications changed recently? Check policy and notifications via Workbench ---> Sys Admin ---> Time Sheet Policies ---> Notifications.
Check the Resource Time Management Settings for the some of the impacted resources. There is a "Do not send this resource time sheet notifications" option on the Time Management Tab for a Resource. See if this is checked.