After connecting the Garmin Forerunner 210 to my PC, it gave me a blank screen and nothing else. After some research I found some reset methods:
Press the lap/reset button a few seconds and then press the light button to switch on the Garmin device again. Pressing both lap/reset and the light button may work better: keep both buttons pressed for several seconds until the 210 watch is “reanimated” again.
Method 2 (tested):
If this fails you can try the hard reset; keep the following 3 buttons pressed together for 15 seconds: page/menu, lap/reset and light. Release the buttons and press the light button again.
Soft Reset: keep the page/menu and lap/reset pressed for a few seconds.