Rootkit, yes or no?

Have since yesterday the following problem. Since yesterday evening, my Huawei 625 U51 does not boot up anymore, but shows only the Huawei sign. Yesterday morning and also the last days, weeks, months, it was still faultless, have it scanned several times by virus scanner, found nothing. What do you think, what is it? Trojans? Other virus type, or even a rootkit damage? Please help. You're welcome. There must be someone here expertly.

It can't be said that easily without knowing you and the device.

Because you can be the cause by installation, downloads or updates - The device may have a defect - etc.

What if you reset the device to factory settings (hard reset)?

A rootkit that prevents your phone from operating? That would be rather counterproductive. Even viruses that just break something are rare. Most of the damaged goods are more likely to pull your money out of your pocket or similar.

You can reset it by hitting the volume down key when you turn it on and selecting "Factory Defaults".