The law varies by state,
county and sometimes city,
but generally it is not legal for the landlord to simply lock you out or take your possessions in lieu of rent.
If you are in arrears he CAN evict you for non-payment (this violates your lease),
but he MUST go through the legal steps in order to do so.
If your landlord has unilaterally locked you out or simply taken your belongings it is VERY likely illegal and you can possibly counter-sue him in eviction proceedings for theft and violation of your lease agreement.
Check with your local Landlord Tenant Board for the exact procedures and laws in your area.