I have no stomach muscles

 It's official.  I have no stomach muscles.  I met with my personal trainer for my very first training session.  She had me lie on my back, but my legs up and put one of those big exercise balls between them, then lower my legs and raise them back up.  I couldn't do it.  My legs felt like lead and I couldn't lift them up.  Oh well, something to work on.  Was able to do some of the other things.  Next week, I'm increasing sessions to an hour because we can't do that much in half an hour.

That's a really hard exercise! In my Ball Pilates class we do the same, including some variations. Try these:
1. Ball between the ankles while laying on your back and feet overhead. Bend at the knees and lower the ball to a 90 degree angle (so your knees are at table top when bent).
2. Same position, now bending the knees all the way so the ball touches the glutes.
3. Ball exchange: with feet still overhead, lift up to grab the ball and extend your arms overhead as you lay your legs and arms down away from your core. Lift the ball back up, bringing your legs back up at the same time, and exchange the ball from hands to feet, lowering legs to the mat and hands overhead to the mat.

Each of the 3 exercises should be repeated in sets of 3, reps of 15.

Just keep working on it - you'll get there.  You've got 'em, you've just got to rejuvenate them!   Don't worry, a little at a time, and you will get there.  Slowly and surely wins the race!!

 Thanks.  That's what my friend who's walking the half marathon with me says.  Just keep putting one foot in front of the other and we'll make it to the end.

 Thanks.  Those sound like something I can do.  Now to make myself do them.  LOL  I guess the gain is worth the pain.

Oh we've all been there some time or another.  I will say once you get them, they still get sore from all the core work, but its a good soreness :).  Keep up the good work.  I do a lot of regular crunches, reverse crunches, oblique crunches, elbows to knees, situps, superman, twists with a medicine ball, leg raises with crunches, and planks are just a few.  You could always add some of those where you can do them when you are at home too.