Hi Christian. I'm Greg, 10 years awarded Windows MVP, specializing in Installation, Performance, Troubleshooting and Activation, here to help you.
Restore the Windows bootloader to recover from Linux Dual Boot:
https://askubuntu.com/questions/1240994/how-to-...
https://diarium.usal.es/pmgallardo/2020/06/13/r...
https://superuser.com/questions/1294071/how-to-...
https://windowsreport.com/windows-10-bootloader... (Avoid ad links which intrude into editorial copy, especially avoid Restoro and Reimage ads)
I hope this helps. Feel free to ask back any questions. Please report back results for all steps given so I know what else to suggest if necessary.
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