Ok, sometimes, if a LiPo drops below a certain voltage, the charger won't charge it.

What I have done... And this is NOT a safe thing to do... is to hook it directly to the 12v battery. BUT, Only for about 3 - 5 minutes and stay right there holding it to make sure it doesn't get hot!

After a few minutes, disconnect it and try using the charger again.

PS, Also, before you try the above, make sure the 12v battery you are using is fully charged. I have an extra car battery in my garage that I would charge LiPos with, and had the same problem as you once - it turned out the 12v battery had run down.