Bichon

Question
at bedtime  my dog goes under  the bed and will not come out.he hides there.
also when I throw his ball in fetch he becomes more & more aggressive about releasing his ball.

Answer
The Bichon is not a retriever.  In fact, male Bichon can become quite dominant and sometimes aggressive/possessive (resource guarding.)  Your dog appears to fall into that category.  If that is the only occasion upon which you see aggressive behavior, don't play fetch with him.

Regarding his going under the bed: your dog appears to be demonstrating (again) a resource guarding behavior.  For some reason, he has chosen to den under your bed and perceives it as his space; whether or not this began as a fear related behavior is not something I can know, not being able to question you.  If this behavior disturbs you, don't allow the dog into your bedroom at any time.  Create a comfortable night time place for him in another area, possibly the kitchen.