Show post titles

Hi there,
I wonder if anyone can tell me how to show post titles? Yesterday I edited the code so that the page titles were hidden, without realising that this would hide the post titles also. However I can’t remember where I edited the code first time around! I wonder if its possible to add a bit of code that will show the post titles again?
I am using twenty twelve. My knowledge of code/css is very limited!
Thanks in advance.