Please follow the steps below to solve this issue. 

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Go to Support link --> SupportCenter Plus application status section --> Scheduled Activities
If the Next scheduled time is greater than the current server time, please restart the application. Check this solves the issue, still issue persists, follow the steps to solve the issue
Please find the schedule id for the 'DocumentIndexingTaskSchedule' process from the above link
1. Stop the application
2. Connect to the database, execute the below queries one by one
delete from INDEXDETAILS;
delete from INDEXDETAILSQUEUE;
delete from task_input where Schedule_id=x;
Replace 'x' with the scheduled id taken from the scheduled activities link
3. Execute indexdata.bat or indexdata.sh from the C:\\ManageEngine\\SupportCenter\\bin 
4. Then start the application.
5. This will solve the issue.
Please refer the below link to connect to the database
https://www.manageengine.com/products/support-center/faq-general.html


Note: 1. Use indexdata.bat --help  to index the data module and year wise..
2. change the parameter to true in indexdetails.txt file from  C:\\ManageEngine\\SupportCenter\\applications\\extracted\\AdventNetSupportCenter.eear\\AdventNetServiceDeskWC.ear\\AdventNetServiceDesk.war\\conf-files folder, if you invoke this process in the application running status
Out of Memory during indexing process:
1. Update the JVM parameters to higher values.
2. You can reduce the loop count to 25 or less and then perform the indexing process
update Globalconfig set paramvalue=25 where category='search';