If the easement is written and recorded, then no, your neighbor can’t generally prevent you from using it. There may be some exceptions depending on the specific facts of your case, such as whether the easement was terminated or abandoned over the years.
And, no, your neighbor can’t cause you to be landlocked. If you do become landlocked, there are various remedies for that situation. But there may be some other things going on here too. It is worth taking a copy of your easement to an attorney and having someone give you some specific advice on your situation.