So if all went according to plan, after you unlocked your bootloader way back in the early days of rooting, you replaced your stock recovery with a custom one, flashed a SuperUser binary, and went along your merry way. Some of you flashed custom ROMs and went deeper into the root world. Both sets of you ended up at a place without a stock recovery installed.
While most of the time you do not need to have a stock recovery on your device, there are times it can come in handy, such as if you’re wanting to go to a Sense-based ROM and download and flash updated OTA firmware. You do this to get new radios without waiting a couple of days, or if you’re S-ON it’s the only way to update radios.
Fortunately are a couple of repositories of HTC One M8 stock recoveries available now over at XDA that should work with most of the HTC One M8 devices out there. You’ll need to know your current software version number for your phone/carrier/ROM, then download the recovery, and install it from a computer with the fastboot command for most of them. There are a few that can be installed via your current custom recovery, also.
Once you restore the stock recovery, you’ll need to replace it later with a custom recovery such as ClockworkMod, PhilZ, or TWRP if you want to do any more flashing.
The two locations you can grab these recovery options at seem to contain slightly different material, so if you don’t find what you’re looking for at one of them, go to the other. Some appear to be TWRP/PhilZ flashable, whereas most are fastboot flashable images.
Just remember, if you break your recovery you’ll need a computer with Fastboot/ADB on it to fix recovery mode. It’s not the end of the world, but it will take you an additional couple of minutes to repair.