If skimmer weir (the flapper gate) has come out or broken, replace it with new. If skimmer line may be clogged, use a plumber's snake to try and break up the leaves. If plastic skimmer is leaking,use an underwater pool putty to patch it temporarily.
Inspect your weir regularly and replace it promptly if it is damaged or broken. If you have an above ground pool, you need to unbolt the skimmer from the pool.