From looking at the picture, I'm not quite sure if the plants are dead or not. I'm pretty sure, however, that they are alive and can easily be brought back to health. The reason why I believe this is because tomato plants are really good at getting better even if they look really bad. In fact, I water my tomatoes when they start to look droopy!
So, first of all, uncover them. You may want to leave the cover on for only the night, only if it gets below 5Â°C (41Â°F).
Secondly, make sure to water them really, really well. Especially now, you may want to soak the soil the tomatoes are in really, really wet.
Also, a quick tip for healthy thriving tomatoes: use fish emulsion fertilizer. This stuff is super good for tomatoes. Basically any fish-related product is good to for them. As a matter if fact, you can even dig fish heads into the soil where tomatoes grow! I used to do that (until raccoons noticed what's going on!) and the tomatoes really benefited from it. A really noticeable improvement in the tomatoes itself was recognized as well as the tomato plant got a lot bulkier and sturdier.
Anyway, good luck and hope your tomatoes get well