People can't hear me on the phone unless it's on speaker/headphones

My dad and brother kindly replaced my phone screen a couple of weeks ago because it was cracked but now the microphone doesn't seem to work..

When I'm on speaker, people can hear me but not if I'm not. People can also hear me if I've got headphones in.

Everything else is fine (except my finger sensor which accidently snapped) and I'm sure I can live with it but it's just very irritating to use speakers or headphones every time I have to make a call. Does anyone know why this could have occurred/how to fix it. Suggestions are very much appreciated :)