If there is anything important on there, back it up while you can. For all you know the next time this happens it might not come back.

The trouble is that there are so many things that could cause this that it's impossible to diagnose it from the symptom alone. But it is possible that this is a hardware fault developing, which is why I say you need to back up your data.

 

sounds like its time to consider getting a new phone. not surprised you are having issues with it......this phone has been out since 2015!!!!

 

Maybe shut it down completely, leave it a while and try again. It the problem is heat related that raises your chances of it booting, if not then it's just a matter of taking a break before trying again (heat-related problems were not unknown with the M9, so this may be worth a shot. If it is that though there is no fix).

I do fear that this is a hardware fault, so if you can get it booted again I'd do a backup as a priority. After that you could try a reset: if it won't go into recovery you could install fastboot on your computer, then connect the phone, boot info the bootloader, then the commands "fastboot erase data" and "fastboot erase cache" would be equivalent to a factory reset. But I'm not optimistic that this will fix this one.

You could try finding the correct RUU for your device (will depend on region, android version, carrier if it was originally a carrier-locked device) and seeing whether you can reflash it, but if it crashes during the flash that's likely to be game over (if it crashes during the flash of the bootloader that absolutely is game over, but the bootloader flash only takes a couple of seconds). To be honest I suspect this is a long shot too.

 

Reinstalling the firmware is what I was referring to when I talked about using an RUU to reflash it ("flashing" being the general term for rewriting the NAND storage that the firmware lives in, and an RUU is a ROM Update Utility, HTC's reflashing package).

There are a couple of catches. The big one is that there is different firmware for different regions, and often different carriers if your phone was originally sold through a mobile service provider. Technically the hardware may be compatible, but the phone will check that the firmware you want to flash matches the region/carrier the phone was originally configured for, and it will not load if not (technically it checks that it's compatible with its "cidnum", or customer identity number - the customer here is the service provider or the HTC regional office, not the end user). So you need to identify your cidnum, either directly (can be done using fastboot) or indirectly (if you know what country the phone was originally sold in and whether it originally was sold as a service provider model then we can use that information to work it out), and hence find an RUU that is compatible with your phone.

A more minor catch is that the RUU cannot contain an older version of the software - you cannot use it to downgrade the software.

The second catch is that you can't find a full collection of RUUs on HTC's site. If you have a US carrier-branded phone you may be able to find the RUU on the carrier's site, or possibly at HTCDev.com, but in general RUUs were not released to the public. So we rely on leaks. There were sites that collected these things, but shipped-roms.com don't seem to have a section for the M9 (codename "hima"), and the other main site has gone offline now (these things were mainly used by people rooting, and both rooting and HTC have dropped massively in popularity). So while a search through the XDA-developers forum will locate some of these things, I can't guarantee that anyone will be able to find one that's compatible with your phone (some never leaked, e.g. if you were on Orange/EE in the UK RUUs never became public).

Assuming a suitable RUU can be found, they come in one of 2 forms. A zip file containing a set of images, which you flash using fastboot (there is a specific recipe for doing this, which I can find easily enough), or a Windows .exe which does everything for you (but you probably need to put the phone into download mode first - normally it would do that for you too, but if you can't run Android on it reliably that's not going to work). Of course the catch with the latter is "Windows" - there is a trick to getting the zip out of the .exe, but it still needs a Windows computer to run on to do that. But that's some way down the line, locating a compatible RUU would be the first step.

Note that all of this stuff with RUUs and cidnums is not generic Android phone stuff, it's HTC specific. Different manufacturers use different approaches and different tools for firmware flashing, and it's only because I used to modify my HTCs' firmware that I know this stuff (partly because before starting down that path I wanted to know how to recover things if I messed-up, partly because I sometimes used the same techniques to flash updates to just part of the firmware).

 

Quite frankly it sounds like time for a new phone.