I am on the lease with her until August. Because of a situation, I made arrangements with the landlord to pay him my 1/2 of the rent directly and I moved out. I am still on the lease. My roommate changed the locks. Does this make the lease null and void for me since I am paying rent on a property I no longer have access to?

No... because as long as there remains a lease... you could be held responsible for the full ammound should they fail to pay. The lease was a binding legal contract with your landlord...not between you and your roomate...

If you moved out... how do you know the locks were changed?

No, your contract is with the landlord, the other roommate changed the lock.
But did so, after you moved out. If you moved out, why do you know locks were changed. Moved out, would mean take everything, and live somewhere else.

If you really moved out, then they could change the locks.
But if you were not moved out, then you can require them to give you a key.
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No, the changing of the locks does not void the lease as the others have noted. W're assuming here that you informed the landlord and/or your roommate that you were vacating the premises and that you moved your things out.

Since you were signed on the lease, you are responsible for your share of the rent until August. You might even be held responsible for the full rent if the roomie doesn't pay.

However, you might be able to get out of the lease if a new roommate moves in. Or if you negotiate a release with your landlord.

Until then or until the lease expires, you could go and demand a new key and move back in.

OP cannot "demand a key" from the landlord; the landlord did not change the locks. However the landlord could probably demand one from the roommate; in which case would probably need to give a copy to the OP.

And I don't know that OP can "demand a key" from the room-mate. Relations between co-lessees such as these are not governed by they typical landlord/tenant act. They are instead governed by the more general laws governing relationships between co-owners of property. In this case, the co-owned property would be the leasehold.