Hey
You can try restarting an ArcSDE service (if applicable) to release possible locks. Thereafter, you can always check if you got any orphaned connections to the service (if not using direct connection).
Close ArcCatalog and any other applications that could consume the sde database you are working with. Use the commands as below:
To show who is connected to an ArcSDE service:
Sdemon -o info -I users -i <servicename> -p <sdepassword>
To kill all connections to an sde service:
Sdemon -o kill -t all -i <servicename>
If you are using direct connection, it is going to be a bit harder.
To kill a direct connect to an sde database:
Ths can be done either by restarting SQL Server database engine or by using SQL Server Activity Monitor, tracing the locked processes, and killing those processes. These things are being done on the SQL Server level, because direct connection has nothing to do with sde services �?? all users connect directly to the SQL Server database engine bypassing sde.
Tell us if you succeed.
PS. Hope you did not compressed the dataset, since compressing datasets in a file geodatabase makes them read-only.