Just as an FYI, in MA the longest an employer is required to hold your job for you, even in a workers comp situation, is 12 weeks, so they haven't violated any laws there. You got more time than legally required.
MA law also states that IF you were out on a workers comp injury/illness; if your employer terminates your employment because they are unable/unwilling to hold your job any longer; if you are subsequently medically cleared to return to work; and if at that time your employer has an opening for which you are qualified, you have to be given preference in hiring.
Note that this is only true in workers comp situation and that even in workers comp situations, they do not have to create a new job for you, bump someone out of theirs, or put you into a position for which you are not qualified. But you have to be given first shot at any positions that are open anyway and for which you meet the stated qualifications.
They can legally tell you that unless you can come back by such and such a date, you will be terminated. They can terminate your employment if you are still medically unable to return by that date. BUT, when you ARE able to come back, IF there is already an open slot for which you qualify, they have to give it to you.

