Unable to retrieve data from Firebase database

There are four problems in your code. The first is the query you are using. If you want to get results based on conditions, change the following query:

DbRef2.child("45961").orderByChild("FromId").startAt("22222").addListenerForSingleValueEvent(/* ... */);

to:

DbRef2.orderByChild("FromId").startAt("22222").addListenerForSingleValueEvent(/* ... */);

No need to .child("45961")call as you need to iterate over the results.

The second problem is that you are taking the DataSnapshotobject as a parameter, but not looping to get the result. To actually get the result you should use the following code:

@Override
public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
    if (dataSnapshot.exists()){
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            Toast.makeText(getApplicationContext(),"hello",Toast.LENGTH_SHORT).show();
            Notification notf = ds.getValue(Notification.class);
            Toast.makeText(getApplicationContext(),"hai"+notf.getFromId(),Toast.LENGTH_SHORT).show();
        }
    }
}

The third problem is that you are not checking for potential errors. You might also consider using:

@Override
public void onCancelled(@NonNull DatabaseError databaseError) {
    Log.d("TAG", databaseError.getMessage()); //Don't ignore potential errors!
}

The fourth problem is that you are using names that start with a capital letter. For this, I suggest you to check the answers from the following articles:

Firebase Android ListView not showing

either:

How to read Firestore subcollection and pass it to FirestoreRecyclerOptions

If the fields in your class are lowercase.