@Megha *******
not to be mean but lets leave the b/s story out of your post so people can give you answers
you entered on a 90 day single entry Non-B and were stamped in for 90 days. At this time you did NOT get an extension on it from the immigration office <- meaning you're still on the 90 day entry.
IF you want to leave and come back you need to buy a re-entry permit at the immigration office or the airport before you fly out (1000baht single re-entry) AND you need to come BACK to thailand before that original 90 day stamp you're here on now expires.
If you can't make it back during that time you need to start the process all over with a new Non-B visa.