Make sure you have USB debugging enabled. If it's already enabled try following
In device manager, does it list your nexus 5 under Portable devices or Android. My guess is it lists it under Android.
What does it say below N5 in device manager? If it shows something like "Android ADB intereface", try following
- Right-click the Nexus 5 in device manager and then click on Update Driver Software.
- Next, click the 'browse my computer' option.
- In the new window click on "Let me pick from a list of devices....".
- In the next window select "MTP USB Device" and click Next to install.