Can't generate apk

Is Developer Settings > USB Debugging enabled on the phone?

Connected Android phone is not a requirement to build apk...

The reason that you get No Android Device Found, is probably that you don't have installed the device driver for that specific phone. If you can't see the phone using windows explorer, then that's the case. So, this is the first thing to fix.

Then, you don't need to build & run: If you choose File/Build Settings, instead of File/Build & Run, you can then click on the Build button (just Build, not Build & Run). Then you'll be asked for a location to save the apk file. Then you'll need to use adb.exe from command prompt, to install it in your phone. You'll find adb.exe inside the android-sdk\platform-tools\ directory. But this won't work if windows can't see the phone.

Leave the phone aside for a while..... still i cant build apk ..but Everything was working fine.... but since i connected my phone ,i cant generate apks any more

Maybe you should try disconnecting it then? If it is already disconnected, then maybe Unity restart or even Windows reboot?

EDIT: sorry - after looking at your question again, I see you disconnected it.