Why does my iPhone 4S say NO SIM when there's one?

My iPhone 4S is saying no SIM and then sometimes it says invalid SIM and the SIM is in my iPhone so why is it saying this and how do I fix it?

Restart your phone. If that doesn't fix it, take a paper clip and pop the sim card out. Then put it back in. Just make sure you put it back in the right way. Last resort take it to an Apple store.

Did you cut the micro sim yourself with scissors? If not just take it to the apple store and you should have it replaced, don't deal with it or stress about it if it has warranty.

You maybe put the sim wrong way?

Its either you may not have put the sim in a proper way or your phone is locked to its network (that's why it says "invalid SIM").In order to use other carriers, You need to unlock your iPhone 4S. This can be done by Remote unlocking. You can get this unlocking service from online sites.http://www.simpleunlocking.com/apple-iphone-4s/rs16wp9/ is where i unlocked my iPhone, Check it out!

Please help my phone keeps on saying no SIM card when I've took it out turned if off and nothing happens?