Boot issue - Dell G5 5090

Hello,

Around a year ago I made a linux USB stick for my PC, I had to change some settings to allow it to boot, but it ended up working as expected. I no longer use the linux USB stick and instead use windows on my hard drive, however my PC cannot recognise my OS on my hard drive. Every time I boot I get taken to Dell diagnostics to fix the problem. To run the windows OS during boot I have to press F12 to select windows boot manager, and then it straight away boots into windows. 

I've tried resetting any BIOS settings and diagnostic checks but it doesnt seem to be working. Tried startup repair and some other windows tools to repair my startup image but it hasnt fixed it. Any ideas?