I had my router off overnight and the IP changed, is it possible to revert back to the original IP address or specify it (I know the original address)
You have a dynamic address that is assigned by your provider. The IP address will change when you turn off the router or at intervals set by your provider (or your router). There is no way to force the IP address to revert back. If you need an IP address that is constant, you would need to get a static address (which providers usually charge more for).
mjewel is mostly right on the mark. Although there is no way to force an ISP to give you a certain IP if they have properly configured an addressing scheme based on mac addresses. You can however attempt to set the IP of your router (modem technically) to your old IP. If this IP is in the un-used pool of IP addresses you might be in luck.