Hi @kaimartin
Can you hear the HDD spin up when you start the laptop?
Start the laptop and get into BIOS and check in ''BIOS > Boot Options > EUFI boot order > OS boot manager'' and check if there's a HDD listed there..
Here's a [https://www.youtube.com/watch?v=y7Pt-10BScU|video] that shows how to get into BIOS to check.
''If you don't hear the HDD spin up when starting and if there's no HDD listed'' then either the HDD has failed or as you suggest it might be disconnected from the motherboard somehow.
Here's the [http://h10032.www1.hp.com/ctg/Manual/c05228250.pdf|maintenance and service guide] taken from this support [https://support.hp.com/us-en/product/setup-user-guides/hp-pavilion-15-au000-notebook-pc-series-touch/model/11830112|webpage] for the laptop. Go to p.32 (42 of 94 pdf) to view the procedure to remove the HDD. Doing this will allow you to check if it is connected securely. If it is then most likely it has failed and will need to be replaced. Hopefully you've backed up your data recently as it may be difficult (and expensive) to recover the data.
''If you hear the HDD spin up when starting and if there is a HDD listed'' then most likely it is a corrupted EFI partition. Search online for how to repair an EFI partition