Why my mac doesn't sense when the lid is closed?

Hi, i'm having problem with my MBP 13'' mid 2012 since i've opened it for changing RAM and clean it a little. I notice that when i closed it, the battery indicator doesn't work anymore, and when i close the lid, nothing happen, like the lid is still open. I read here on iFixit that the sensor for the closing lid is part of the battery indicator. Later in the day i'm gonna re-open it and see what happened.

What could be the problem? Is it possibile that i disconnetted it accidentally? or it's probably broken?