Welcome to Manjaro!
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Please read this:
[HowTo] reboot / turn off your frozen computer: REISUB/REISUO -
Please read this:
[HowTo] become a Manjaro power user when you're a wizard at Windows but a N00b at Manjaro / Linux
(Long article, but the above link points you to the right section and maybe the other sections might cach your interest too!
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After that, delete all the Stable Development kernels and install 5.4 as an LTS fall-back.
Problem solved!