iMac will not boot into anything

As the title says, I cannot boot my iMac (late 2012) into anything. I’ve tried everything from booting into Safe Mode, Recovery Mode (and Internet Recovery Mode), doing a diagnostics test (which didn’t work), and resetting NVRAM and SMC; nothing is working. I’m just stuck in a boot loop, the progress bar gets to around 60% and it turns off and reboots. I also have an external hard drive which I boot into to play games but when I try to boot in using the Option key, I get back to the boot loop. I’ve tried booting into Single User Mode and trying to run fsck but I can't do it because I can't disable SIP. I’m really stuck on what do to. Is there any fix to this, or do I have to take a trip to the Apple Store?