If it costs a lot less to replace a window that you can climb through then do that. I wonder what kind of landlord doesn't have a key to his properties? You really need to make a spare and he needs one as well in case of emergencies or maintenance when you aren't there ....that's how it is done here.
(UPDATE) i spoke to the landlord and he said he only had the one key because it was a temporary lock as the old one was kicked in. he's aranging to get the locks changed n i'm at a relatives place for the now :/
I don't know how you'll do it without a locksmith: but if you do, make sure you phone round: prices can vary greatly. You should be able to get it for 30 with a bit if luck.
Find someone who knows how to pick a lock...that or jimmy a window open and climb in that way. I've locked myself out of the various places I've lived so I feel for ya!