These days, the typical life span for a work of fiction is measured in months: that means that your book, unless it's selling very well, will one day probably be off the shelves and out of print.
You'll feel sad, of course, but there is something you can do to prepare for this:
First, check the out-of-print clause in your contract. Chances are that rights revert to you after a specified period of time. You may have to request that reversion in writing. Once you have the confirmation of owning the rights again, there are things you can do with those rights.
The best thing. of course, is to have your agent look for a new publisher. If that fails and you have a reasonable marketing plan for th ebook, you might consider placing it with one of the many Print-on-Demand publisher (Xlibris is one). You should also explore the programs available through Amazon to make your book availabe electronically as a PDF or e-book.
Just remember: like the man said'it ain't over 'til it's over.