My phone has a white M on a black screen. I can't reboot.
Welcome to the forum Joe. I will move this to the rescue squad to get you some quick help. They are going to need more information tho. What were you doing that would cause this? Were you trying to root? What method were you using? Good luck
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Since you didn't give any information, we really have no idea what your situation is.
A simple start will be to pull your battery, put it back in, then turn it back on.
As stated, pull the battery, leave it out for 30 seconds or so and power back on. Then say what you have (i.e. still have the M bootloop, back in service, etc.). After that just give a brief description of what you (they) were trying to do.