OK - I had the same problem in that the watch went completely blank - nothing and could get nothing to work. Had it plugged in for 72 hrs to charge - nothing. Your suggesting of pressing and holding both…
Eureka! It works! Holding the Light and Select buttons simultaneously turned on the otherwise lifeless 235. It went blank when I plugged it in for charging into a USB port on my Intel NUC PC. Never happened…
Had my 235 "black out" just a few days ago, and so came here to find the fix/solution. Yes indeed, simply hitting the upper left and upper right buttons concurrently worked!