If you have a valid backup then restore the database otherwise try below steps:
Run a backup and restore this backup as a new (test) database; then run a DBCC CheckDB with one of the "Repair" options: repair_fast, repair_rebuilt, repair_allow_data_loss on this new test database to see if it could be fixed by SQL Server.
Use database_name (Test database name)
ALTER DATABASE database_name SET SINGLE_USER
DBCC CHECKDB('database_name', repair options)
to return to multi-user mode:
ALTER <database-name> SET MULTI_USER
After that run DBCC checkdb again to check if error is fixed
If this test was successful you could perform the same on your productive database, of course during an "offline" time while no one uses the database.
If it fails to fix the problem then you would try SQL recovery software from here: