STICKY: Is your Pi not booting? (The Boot Problems Sticky) - Raspberry Pi Forums
If you are reading this, you are presumably having trouble getting your Raspberry Pi or Pi 2, PI3(+) or PI4B to boot. This post will talk you through the potential causes of your boot problems, and will offer some solutions. If you use the PI3B+, note that it will only boot past the "rainbow screen" if you feed it the right (latest) boot files. So in case of trouble try using the latest Raspbian from the download page, or try updating your older software on an earlier PI on which it boots, with Raspbian that should work. If not ask the people behind the software for a compatible version.