I second what Ian says. You should just change the nameservers.
If they're also your registrar, you should probably contact ICANN.
Actually most hosting companies, if you registered the domain through them, they are just a reseller for a larger registrar.
So it pays to do a whois check and find out who the registrar is, and contact the registrar direct as they will more then likely help you if your hosting company went out of business.
If your domains is with a registrar and they were to go broke, which does not happen all that often, then you would need to contact ICANN, but they usually see to it, that arrangements are made to take over management of registrars who are out of business.