i made an offer on a home and was accepted and signed by seller.a week later the seller canceled saying they had a 2nd mortgage they were not aware of

In a practical sense, you're going to have to find another home that you really love. In a legal sense, if you want to try to force the seller to sell, you'll have an expensive uphill battle and I don't suggest it.  To sue for specific performance, you'll need to tender payment which is difficult to do.