trouble uploading to arduino uno

Using Arduino IDE 1.x
1

i am running win10 pro on surface pro4. arduino was working in past but now i cant seem to upload to arduino.
in IDE tools>port:"COM8">com8 is selected..
the err msg is:
<
avrdude: ser_open(): can't open device "\.\COM8": The semaphore timeout period has expired.
/>

please help me kind folk!

2 
avrdude: ser_open(): can't open device "\\.\COM8": The semaphore timeout period has expired.

My first guess would be that some other app has that serial port open. Try restarting the system.