if there's a way to get to your boot-up menu, then perhaps a low-level cache clear is in order. Google it.
otherwise, you may have to backup your personal content, and then perform a Factory Data Reset. That is a destructive operation. You want to try a low-level cache clear first if that's possible on your device.
I also have the HTC ONE M8 and have major problems with the upgrade. The operating system itself is defective. I've lost apps I paid for and contacts and videos of my son's wedding, all of which were backed up to the cloud server. I used the tech coach service which is a premium/paid for service offered by Verizon and they had me do a factory reset. The factory reset did not revert the phone back to kit Kat, lollipop is still on my phone. Verizon has been advising customers to take their phones to best buy to have them reverted back to kit Kat. I would advise you to back up all the files and contacts and apps to your computer , use the micro jab wire to do so. You can either take it to best buy to revert it back, or wait like I'm doing, since I've already lost files and contacts and apps. The lollipop operating system should have been better tested prior to being "forced" into our phones. I wish you good luck with it.
I found a fix that seems to work, finally! Try going to the Google play store and search for Google Android WebVIew, and Update that app. I just did that and my crashing apps now are working fine *so far* Let's cross our fingers this helps.