What kind of seedlings?

Onions and broccoli will be find, they won't even notice. Peppers and tomatoes will hate you for a week, but they won't die. I don't know about tropical plants. Tubers in the ground should be ok (canna, dahlia). I store them in the garage over winter in zone 5. Tell your weatherman to hold off on the frost till next winter.

It depends on their cold hardiness. If you have things like cukes and melons in already they could croak, as could tender herbs like basil. It also depends on how much wind and moisture comes with that 45 degrees. You can check cold hardiness by looking at usual planting schedules online. Anything that can go in before the date of last freeze will be fine, but to be safe you might want to baby anything that isnt supposed to plant out until after threat of frost (45 isnt frost, but its fairly cold and I like to be on the safe side). if your garden is small enough, you can always throw either some warming garden cloth over them at night, or if you dont have that, even an old blanket (so long as it isnt heavy enough that it will crush tender stems, or you put up some supports for the blanket's weight like a make shift hoop house)

I have basil, mint, hot pepper, cosmos flowers, and beans.. Should I just bring everything inside at night? Only the cosmos flowers have sprouted..

From what i know, which isn't much, there shouldn't be any problem. Last season i throw a couple of tomato, corn, and sun flower seeds and they were fine even though the temp. dropped to the 40's.

If you are talking about bringing them in - they must be in pots? Which makes it easy unless you have hundreds. Dipping down to 45 isn't a problem, two full days at 45 may make them unhappy. The peppers and the beans are the only ones I would worry about, they definitely like warm weather.

When I get to the point that my seedlings are out on the porch hardening off, if it is going to get below 45, I bring them in.