Just check the error log it is necessory that yoou should know the fault. It may be window acceleration HAXM error as mentioned above you have to install it separately in that case... generally the path is c:\users\youruser\appdata\local(run some bat file named like HAXM)
here the error pops in android studio is and above was the solution to this .. \android-sdk\tools>emulator: ERROR: x86 emulation currently requires hardware acceleration! Please ensure Intel HAXM is properly installed and usable. CPU acceleration status: HAX kernel module is not installed!
tell me further if the issue persists altough after this i suppose you will be able to see nexus 5 emulator on screen