Hi Simonr1
Have you installed any new hardware or software since this started happening?
Can you try performing a system restore before this started happening?
Press Windows key + R
Type: rstrui.exe
Hit Enter
This will start the system restore wizard.
Choose an earlier point in time.
Check if you can then access the drive.
Disclaimer: Ignore when you are prompted to give a rating (Yes or No) until we actually arrive at a solution that works for you.