Is having your power turned off justifiable cause for having your landlord change your locks?

Does the lease require that electric be paid? If it does, then technically you have breached the lease by not paying it. Nevertheless, the landlord cannot enter and change the locks and proceed with eviction without proper notice. Now since you were out of town, he may not have been able to contact you to provide proper notice, so that is an issue. But with the electric being off, that probably constitutes an emergency situation, in which the landlord does have the right to enter and guarantee the safety of the property. You need to be in communication with them now, and ask them to justify changing the locks and ask them if they plan to evict you.