I need to migrate this database to another instance of SQL, but after I migrate the database, I cannot decrypt the information.
You have a database master key (DMK), which is protected by both the service master key (SMK) and a password. Since you're migrating to a different instance of SQL Server you can still access the DMK through the password, however automatic key decryption will no longer work because the SMK has changed.
The fix is easy enough. When you restore the database to the new instance, run the following:
open master key decryption by password = 'MyPassword'
alter master key drop encryption by service master key
alter master key add encryption by service master key
close master key
This will drop the encryption of the DMK by the SMK, since it can't decrypt it on the new instance. Then it will add the encryption through the SMK back to the DMK so that automatic key decryption will work.
The reason I don't say to backup and restore the SMK, as the other answer shows, is that it affects all of the databases on the instance whereas just fixing the broken one won't cause any other issues on any current databases.