It sounds like your door spring has either become unhooked or broken. This is generally due to normal wear and tear not only from use, but also from the heat of the toaster itself. If this is the case, you would have to replace the spring. Fortunately, replacing the spring is a fairly simple repair process. There is a section in the guide for this repair check out the troubleshooting page. Black and Decker TO1303SB Door Spring Removal/Replacement

Unfortunately, this is a common problem with this toaster oven. There is no way to repair the handle itself. The door will need to be replaced completely. You can refer to this guide. Black and Decker TO1303SB Door Replacement