You have a bad boss. You may also be a bad employee. (What do you mean no new assignments? What do you spend your days doing??)

1. Update your resume. You should do this regularly even if everything is fine.

2. Ask your boss where you can help out, since you seem to have some time. That's all. Don't confront him over being ignored or any such thing. Just offer to help.

3. Start checking out other jobs you could apply for. You don't have to apply but you have to be ready to do so when the need arises.

4. If nothing else changes, your next review is likely to be terrible. Change things before that happens. Whether you change this job or change to another one, don't wait until you're angry, or worse, unemployed.