首先，在命令行窗口中输入'adb devices”命令，如果没有任何设备列表，则可能是由于Android模拟器未完全终止或操作系统出现问题。在这种情况下，请尝试手动终止模拟器进程，并重启操作系统。其次，可以尝试在ADT中启动模拟器，但在启动前先选择'Wipe User Data”选项。最后，检查操作系统是否需要安装任何更新或修补程序，并确保您正在使用与模拟器兼容的操作系统版本。以下是Java代码示例，显示如何启动Android模拟器：

import com.android.ddmlib.AndroidDebugBridge; import com.android.ddmlib.IDevice; import com.android.sdklib.AndroidVersion;

public class EmulatorLauncher {

public static void main(String[] args) {
    AndroidDebugBridge.init(true);
    AndroidDebugBridge adb = AndroidDebugBridge.createBridge("/android-sdk/platform-tools/adb", true);
    IDevice[] devices = adb.getDevices();

    // check whether any emulator is already running
    for (IDevice device : devices) {
        if (device.getAvdName() != null && device.getAvdName().startsWith("MyAVD")) {
            System.out.println("Emulator is already running.");
            return;
        }
    }

    // launch emulator
    AndroidVersion androidVersion = new AndroidVersion(21, null);
    String avdName = "MyAVD";
    ProcessBuilder pb = new ProcessBuilder("/android-sdk/tools/emulator", "-avd", avdName, "-skin", "HVGA", "-verbose", "-no-boot-anim");
    try {
        Process p = pb.start();

        // wait until the emulator is started
        while (true) {
            IDevice[] newDevices = adb.getDevices();
            for (IDevice device : newDevices) {
                if (device.getAvdName() != null && device.getAvdName().equals(avdName)) {
                    System.out.println("Emulator is started.");
                    return;
                }
            }
            Thread.sleep(1000);
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
}

}