More reasons to work with a trainer include:
You are recovering from an injury: A professional will design the right workouts to get you back in shape, without aggravating your previous injury. This is an essential step if you are to recover fully and avoid a pattern of repeated injuries.
You struggle to eat properly: Many people work out hard, then go out and eat junk food. Working with a professional trainer or nutritionist can help you get a handle on your diet.
You are pre- or post-natal. A trainer with experience in this area can give you the expert advice you need to maintain your conditioning during and after pregnancy.
Before you commit to working with a trainer be sure to find out what experience they have - particularly in the area you need help - and get some references from previous clients.
And remember, this does not have to be a full time commitment - even three or four sessions with a qualified trainer could give you the kick-start you need to get your workouts back on track.