What does it mean when my iPhone 3GS says no sim in the top left
When your phone says no SIM in the top left, the problem is just that your SIM card has become slightly dislocated. This may cause your iPhone to stop connecting to 3G networks and even avert you from making or receiving calls.
Better be careful, dude.
Theres a little slot at the top of your phone in might have slightly raised up if you droped it so just push it down or try turning it off then back on