I don’t know what you mean by contract, but an employer can always tell an employee they can no longer access the employer’s facilities (or log in remotely) anytime, barring an extremely weird agreement giving an employee that right.
And outside of Montana, employers can stop paying employees immediately barring an agreement that the employer cannot do that. This is called at-will employment. The most common agreement that typically does not allow an employer to do this are usually called collective bargaining agreements, negotiated by unions.