Same thing happened to me, but it wasn't an accident. Now let's see...
Can I reset my phone to factory settings?
Yes. I had totally forgotten that I've deleted the default launcher when I resetted my phone, so it was just a blank. However, there's a trick you can do. But how are you going to download a launcher when you don't have one? How are you going to even open Play Store? Well... You can still open the notification bar, thus you can access the settings app from there. When you get to the settings, go to the Apps section. Find Play Store. You can open it from there. Then download the launcher you want, there you have a launcher. So it is a bit scary when you don't know about smartphones but, you can still make it.
Is there a way to download my old default launcher?
Search the internet by the phone's model. If you can't find it, you can't download it of course. But there's still something you can do. I don't remember the name of it but there is an app that can turn all of the apps to an apk file. So if you know someone that can help you with that, go and ask him to download that app, turn the default launcher to an apk file, have him send it to you. But I don't know how you can integrate it to the system so that whenever you reset your phone to the factory settings again it will recognize it as the system's default stock launcher.