I recently bought a Schwinn from the 1980's. The brake cables were unhooked at the handlebars for shipping. I don't know how to get them reconnected again.
It would help to know what kind of bicycle and brakes we're talking about here, but regardless, you need to create slack in the cable so that you can pull enough inner cable out of the housing to hook the ends of the cables back inside their holders in the brake levers. The holders are usually slotted. So, once you have some slack and have some inner cable protruding from the end of the housing, you can feed that end of the cable into the lever and hook the end into its holder in the lever. On a 10-speed the holders will be inside the brake levers. You will probably be able to look down through the hole on the top and see them. On a 3-speed or mountain bike, the holder will be visible when you squeeze the lever. And, may need to feed the lever through the adjustment barrel, turning it to align the slot first so you can get the cable through.
To create the slack, you can try just squeezing the brake with your hand. If the wheels are in the bike that might not create enough slack. If that's the case remove the wheel and try again. If you squeeze the brake and hold it
with one hand you can fish the cable into the lever with the other. Or, use a strong elastic band to hold the brake together so you can use your other hand to feed the cable into the lever.
You might wonder why you don't just detach the cable at the brake to free the cable completely and not have to deal with the brake spring. The reason is that then you will need to readjust your brake. Also, the less you loosen and tighten the cable anchor bolt the less risk of fraying the cable and stripping the bolt, which tends to be rather delicate.
Hope this helps and if what I described doesn't sound right, it's probably because you're working on a different type of brake. Let me know the details and I'll try again!