It boots, shows the logo, and restarts in a loop.
A corrupted update, degraded storage, or a broken system app.
Recovery mode → clear cache first. If that fails, factory reset (we tell you data is lost). If that fails too, it's hardware.
It depends on the model and on what we find inside, so we quote each repair individually. The diagnosis fee depends on the device, and you approve the exact repair price before any repair starts. If it can't be fixed, you only pay the diagnosis.