Hi Manish,
Functionality was working before and now not working ... Does it mean if there has been any changes related to this functionality moved in the system.... Please try checking same in Test & Production system.
Also check if there has been any changes done in email id for specific responsible persons. You can involve Security / Workflow team to check this.
Have a look on below mentioned thread as well
Triggering a Workflow from Milestones in Project systems
Regards
Tushar