Hi Girish,
As Keith said, simply reassigning the access controls for self-service on the respective task will fix the issue.
If you want to do it through table MXP_TaskAccess, following is the way.
1. Get the Task ID of the self-service task.
.
2. Login to the Identity center database with user mxmc_admin and corresponding password.
3. Execute the following select query with the self-service taskid ( task id from step 1).
select * from MXP_TaskAccess with(nolock) where TaskId=<yourtaskid>
4. Check the value of the column TargetMskey for the self-service task.
The value should be '0'. If you find the value is NULL, update the value to 0.
6. UPDATE MXP_TaskAccess SET targetmskey=0 WHERE TaskId=<yourtaskid>
All the best !!
~ Krishna.