If the plants have only lost a few leaves at the very bottom, it might just be normal aging. But if there are a lot of leaves gone, something else is going on. Are the plants in pots or in the ground?

Look for brown spots, dark speckles, or discolored yellow areas on all the leaves that aren't dead yet. Check the stems and fruit for black specks or brown patches too. Then look for the thread in this forum called "HELP!!! Tomato Problem Solver 2" and compare the pics in the links there to your plants, see if anything matches.

Also look for wilting at the tops of the stems when the sun is out, and if you see any, check again in the early morning and see if the wilting went away overnight.

Here is a link that might be useful: HELP!!! Tomato Problem Solver 2