FaceTime app is not working of my iPhone

hi sir / madam

I’m using an i phone 6 model no MQ3D2AE/A software version is 12.5.4

the problem was that in the last few years to not working to me of the FaceTime app. at the time I didn’t want to that importantly. But now I want to importantly

1)what will happen to that my FaceTime app

2)why didn’t not working to me

3)how did I get back to use