This seems to be a common problem with Turnigy LiPo's.
To do this you will need:
Decent soldering iron and solder
Sharp knife
When running, the car will suddenly loose power, it appears that the connection is broken somewhere.
Check your LiPo's with a voltage checker, and a 3S LiPo may now only count two cells, and the voltage across the output cable of the LiPo is 0V.
If this is the case, then most like this has happened.
WARNING: Do this repair at your own risk. You WILL void any warrenties your LiPo's have (although not really a problem as you most likely got them from Hobby King and cba to send them back). Also if you short the LiPo while soldering, you could blow up the LiPo, so like I say, DO IT AT YOUR OWN RISK.
So then, this has happened to my friend a number of times, and so far in just 4 months it's happened to me twice.
My 3S LiPO is reading 0V across the terminals, and only showing as a 2S pack.
First, you need to cut open the casing around the terminals. Do this carefully to avoid puncturing the cells (if you do puncture a cell, get rid of the LiPo QUICKLY!)
I advise you to only cut one side of the casing open, so you can just fold the part that covers the contact over to one side, this is for ease of reassembling the pack later.
After you have cut the casing open you will expose the connections and 90% of the time, this will be the problem.
As you can see, the solder is quite thin from factory, and has to take alot of current. This has caused it to explode, breaking the connection.
To fix this, simply solder over the gap using a generious amount of solder. Be VERY careful not to bridge any neiboughring contacts, otherwise you may short the LiPo. (i've done it, scared the crap out of me )
Now when soldering, make sure you keep the iron on long enough so all of the joint melts, so it's soldered all the way through. You don't want to just bridge over the top of it, otherwise you will have a bad connection and it will just break again.
Please excuse my bad soldering
Also, to reduce the risk of this happening again you can put strands of wire in the joint as TiM mentioned
As you can see, the terminal is now reconnected.
Success, the pack now reads at the correct voltage, and shows 12V across the terminals.
Now just tape up the pack using some electrical tape over the contacts, then wrap it over in gaffa tape.
If you didn't remove the original casing, just fold the flap back over and tape it up.
Job done