I'm just trying to configure my DNS server to switch the DNS entry in my DNS server (bind) if a server becomes unreachable. Is there a simple tool that I can use to do so?
You should be aware that your idea is not ideal. You would have to make a trade-off between the TTL records (the amount of time your DNS records are cached on other servers) and the amount of time the fail over server is not reached by visitors using cached information when the primary server went down.
If you set the TTL values low, a switchover can happen almost instantly, but every request by a new visitor to your server will be unnecessarily slower.
I would suggest using another kind of fail-over, eg. a proxy or fail-over IP.