Could not open avd_name.avd/cache.img

I had the same problem already twice since I upgraded to Android Studio 2.0 on Windows 10. I use a preferred emulator in the Run/Debug configuration. Changing the Deployment Target Options in the configuration to Target: Show Device Chooser Dialog solved it for me. After choosing the same emulator instance in the dialog it started with no problem. Then I change back to the preferred emulator setting. Until the next time it breaks...

Hoping this helps.

Keep an eye out for a background qemu-system-i386.exe process running in the background that will be locking the cache.img file.

This might be still running from a previously failed emulator launch. Kill that using Task Manager and start again.