My roommate moved out over a month ago now, and refuses to pay bills for gas and electricity. She didn't sign a lease but the landlord knew she was here and collected the rent from her monthly. What can I do to get the payment? Is it my responsibility or the landlords?

Who is being held liable for the bills, whose name is the bills on.

It sounds like they are in the landlords name?? If so if you pay him your 1/2 of the bills what will he say or do ?
In the end if you are liable to the landlord for the bills, as stated in your lease, and this person was merely subleasing from you? You may have to sue them in small claims court.

It sounds messed up, if you have a lease for the full builing, and he rented out part of it to someone else, and he collected the rent from them. Not you getting the rent from them and then you paying him,
It sounds like you needed a new lease to show you only liable for your 1/2

The bills are in the landlords name. When my roommate moved in, I asked him to bring a new contract, but he didn't. So now he wants me to pay him but my roommate won't pay me her part. She told him she was moving out, but didn't tell me when she left. So I thought it was between them, and the landlord shouldn't be asking me.