Hi. 
Give it a good 'knock' on the back (hard surface). You might hear a clink sound. I think it's when a cog gets out of line. My chair is fine now. You'll notice that the rollers start moving back to the off setting. So if it goes off and they are in the middle or top then it's stuck and needs a thwack! 

You can also open the unit and move the rollers assembly with your fingers a little to get the whole massage unit back on track. Make sure its not plugged in. Smoothsn