My string of 50 c6 type led X-mas lights don't work. They are only 2 weeks old. Wiring problem? How to tell?

If it is a relatively easy task, can the string of lights be removed and returned to the store where you purchased them?

Yes, I have my receipt and they are going back but, I've noticed a few of the bulbs have a different style base (on the same string) they physically won't fit in the other bases on the string, why is that I wonder?
I think it's possible that more then 2 are burned out, causing the string of 50 to go dark. Most strings only come with 2 replacement bulbs.