Hello and welcome to Android Forums. Hopefully I can help you with your problem.
Try pressing the power button. That will usually light your screen back up during calls. Then just tap the speakerphone tab.
If your phone has a proximity sensor, there is a chance that it has either been disabled or has gone bad. Clean your phones screen very well in case something is covering the proximity sensor.
Have you installed a screen protector or protective phone case? That may be affecting the proximity sensor too if you have one.
If you have any other questions regarding your device, I invite you to join Android Forums. We have many sections for specific devices which contain a wealth of information.