Escobana is right: tread carefully and do some research beforehand.
There's no evidence that 'lost' memories can be recovered. A trained therapist can help you piece together memories that have been splintered due to trauma. However, those memories are not 'lost'; they're just disorganized. The therapist can help you reassemble those memories so that your brain can process them correctly.
Also, we lose memories over time. It's a natural part of how the brain works. Once they're gone, they're gone.
There's a real danger in the hypnotherapist planting memories into your mind. You'll experience and believe them as real - even though they never happened. The therapist can do this even if they have the highest of ethics and the best intentions.
Before you put yourself in such a vulnerable position, know what your goals are and the credentials of the therapist. If you still want to do it and feel confident in the therapist, it can be a big help to your mental and emotional wellbeing.