Publisher (online, refereed journal) is not publishing the volume containing my paper - what are my options?
Can you find a telephone number to ring them? If not, you write an email and state that, if there is no response until [some reasonable time frame], you assume that they are not intending to publish the work, and having invested considerable time in the development of the paper, you will consider yourself fully entitled to publish the paper elsewhere. 

Note that I do not know what the legal situation is (this is not the place for legal questions).

However, as per Dirk's suggestion, I add that universities have a legal department that might offer legal advice on such topics, as long as OP is employed at one.