PC won't boot to Windows 10 from SSD

I bought my PC years ago with only a HDD. I added a SSD myself afterwards. When I booted my PC after a hard shutdown/electricity fallout I had only my HDD in BIOS. I then boot from HDD, get an error and let the PC reboot. After a few times the SSD would be recognized in BIOS and I could change the SSD to default.

I got tired of this proccess a few days ago and found a tip to disable the hdd-partition with diskpart. The first time I had a hardreset again (mouse and keyboard froze and pc didn't shutdown with a single press on startbutton), I couldn't do the HDD boot-trick and tried changing the cables from hdd to ssd (ssd in sata1). This didn't help. In diskpart I made the SSD partition active. I now can see the SSD in BIOS, but booting gives an error "An operating system wasn't found.". When I retry diskpart, the partition could be made active again. It seems like this resets..

Bootrec.exe /FixMbr was successful

Bootrec.exe /FixBoot gives no access error (there could be some parts left of a dual boot setup)

chkdsk gives no problems found

Mediatool boot:

• This PC can't be recovered with start-uprepair.
• You need to choose which installation of Windows you would like to put back when using Systemrecovery.
• Reboot, select a operating system and select Systemrecovery. No systemcopy available.

How could I make my pc boot again?

PC: MEDION AKOYA P5367 D EU (MSN: 10015458)

Motherboard: MS-7728

BIOS version: E7728MLN.209

SSD: Kingston SV200 (128GB)

HDD: ST1509DL003-9V (1500GB)