Reason: This exception occurs when the user is trying to allocate an SGA or PGA size more than what is allocate to the database. This may also occur when the user is trying to allocate the database less memory than the minimum requirement.
1) First create a temporary pfile from current spfile:
2) Sql command : Create pfile=<filepath>/filename from spfile;
3) Open the pfile and edit value of MEMORY_TARGET and set to required value, then create spfile from this modified pfile:
4) Sql command: create spfile from pfile=<filepath>/filename
5) Restart the database using SQL command: Startup