My MacBook Pro kernel-panicked with this message, "You need to restart your computer. Hold down the power button until it turns off, then press the power button again."
The same message appears each time I reboot.
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This sounds as though your system is in a cycle of kernel panics. The first thing to do is to try starting your system in Safe Mode to clear out the system- and application caches and to rebuild the font libraries:
If that doesn't do it, I would next try resetting your PRAM. There may be some boot settings that have gotten messed up. To reset your PRAM, turn off your computer and then turn it on again. Immediately press command+option+p+r until you hear the start tone for a second time, then release it.
If a Safe Boot and PRAM reset don't fix things, you can next try resetting the System Management Controller (SMC). Instructions vary depending on the system, so see the following Apple article for details:
Finally, if that doesn't do it, you'll need to run Apple Diagnostics to determine whether you have a hardware fault. Again, details differ depending on the system, so see the following article:
Best of luck!