Hi Julie , what a shock for you to find Squeak like that, I'm so sorry for you.
You say she was by the food bowl, if you can check her mouth, it's just possible she may have choked. I take it there are no signs of her being in a recent fight, so it maybe a genetic problem.
I had an autopsy when my Sue died, as I wanted to be sure she didn't have an infection. I think it costs around £170, and is arranged through your vet.
When I found Fudge dead on the top of an aviary it was such a shock. She looked in perfect health and she had been mixing with the others for years. I didn't have an autopsy for Fudge as I was sure there was no disease. As Tarcus said it's very sad, but sometimes they do just die suddenly.
If Squeak was your dominant chipmunk you may find one of the others being more lively as the dynamics of your group are bound to change.