A few things I would consider are:

How close to the original exposure is the photo and what determined the 3 main factors of shutter speed, aperture and iso? If you shoot in manual mode you will have chosen these settings yourself (if you set these yourself I would suggest increasing the iso to at least 3200 and your shutter speed to about 1/30th (tell the subject not to move much or shoot at iso6400 and put shutter speed to 1/60th if subject isn't quite still). I'm assuming you didn't shoot in manual and you have shot in either aperture priority, shutter priority or program mode or indeed another automatic mode. At which point the camera tries to do it's best based and set these settings according to the cameras metering mode you have chosen.

My guess is that you have not spot metered on the subjects face and the bright light in the background has caused the metering system (either centre-weighted or matrix metering) to underexpose the rest of the photo.

Solution: either apply exposure comensation, probably between +1 or +2 stops or change the metering mode to spot metering and when the focus point focuses and meters of the subject it should force the camera to shoot at the correct exposure. I would also check to make sure your exposure compensation isn't set below 0. Also make sure your iso isn't fixed at 800 otherwise the metering system will not be able to gain a correct exposure without lowering the shutter speed to a level that's not suitable for handheld portraiture. Sorry I've just typed this up quickly, if anything doesn't make sense my main advice would be learn about the metering system on your camera and double check if your camera has an auto-iso feature as this would help your cameras metering system in these lower light situations. thanks

A few things I would consider are:

How close to the original exposure is the photo and what determined the 3 main factors: shutter speed, aperture and iso? If you shoot in manual mode you will have chosen these settings yourself (if you set these yourself I would suggest increasing the iso to at least 3200 and your shutter speed to about 1/30th (tell the subject not to move much or shoot at iso 6400 and put shutter speed to 1/60th if subject isn't quite still). I'm assuming you didn't shoot in manual and you have shot in either aperture priority, shutter priority or program mode or indeed another automatic mode. At which point the camera tries to do it's best based and set these settings according to the cameras metering mode you have chosen.

My guess is that you have not spot metered on the subjects face and the bright light in the background has caused the metering system (either centre-weighted or matrix metering) to underexpose the rest of the photo.

Solution: either apply exposure compensation, probably between +1 or +2 stops or change the metering mode to spot metering and when the focus point focuses and meters the subject it should force the camera to shoot at the correct exposure. I would also check to make sure your exposure compensation isn't set below 0. Also make sure your iso isn't fixed at 800 otherwise the metering system will not be able to gain a correct exposure without lowering the shutter speed to a level that's not suitable for handheld portraiture. Sorry I've just typed this up quickly, if anything doesn't make sense my main advice would be learn about the metering system on your camera and double check if your camera has an auto-iso feature as this would help your cameras metering system in these lower light situations. thanks