There's more than one solution to this.
I think the best one that I've come across is this one, supplied by Mike T. (well done!):
Step 1: Turn off your phone.
Step 2: Now hold and press both “Power and vol+” button until the “S2” screen Shows.
Step 3: And Press Power Button (Only single press – Do not press and hold)
I've used this method twice now.
I understand that another solution is to take out the battery, then put it back in.
I have not tried this method to sort out the 'safe mode' problem - but I have employed this course bof action when confronted with a 'frozen' screen (currently also twice).