Lab gone astray - bad behavior emerges at 8

Our dog has just turned 8 years old. She has always been very well trained/house broken.
   
Recently (last few months) she has started getting plates, place mats, tablecloths, food off of the counter/table when we are not at home. Today, I came home and she had destroyed and eaten 2 packs of english muffins that were still unopened. I am not sure what to do with her, short of getting a kennel and making her stay in there when we are not at home.
   
Please let me know if you have any ideas and what I need to do to help her change her bahavior for the better.
   
Thanks,
   
L

Has anything changed in your life? Is your dog being left alone for longer than he used to be? How often and for how long do you walk your dog?

The only major change is that our vet told us to put her on a diet. She acts as if she is starving, but we have since increased her food again. The bad behavior started right after the food change and it is almost like once she started to go for it, well - a whole new behavior was formed. Just not sure how to correct it.
   
Thanks,
   
L