Redmi Phone Keeps Rebooting Between the Logo and Fastboot Mode

This sounds like a boot loop, possibly caused by a stuck volume button, a case pressing the buttons, corrupted system files, or a battery problem. Remove the case and check that the volume buttons move freely. Try holding the Power button for 15–20 seconds, or hold Power and Volume Up to enter recovery mode. If recovery offers a cache-clearing option, try that first. A factory reset may fix corrupted software, but it will erase your data, so use it only as a last resort. Since no recent update was installed, hardware or button issues are also worth considering.