The responsibility for access, maintenance, and repair of an A/C is outlined in the governing document of the association, most likely in the Declaration of Covenants. If the A/C unit is a Limited Common Element it is the responsibility of the unit owner, and therefore the unit owner should be allowed access to the A/C unit. If the unit is on the neighbor's property, The owner needs to either have it moved (with the HOA' written permission), or make arrangements with the neighbor when they need to access the A/C. If the neighbor refuses, the HOA board has the duty to enforce the covenants against all members. If the board refuses, they may need to be voted out and replaced with board members willing to enforce the covenants. A demand letter may help teach or remind the board of its duty to enforce the covenants.