|Your PC wouldn't change drive letter assignment by itself. Tell us what exactly lead up to this. Did you install or re-install any operating systems?|
Go into Disk Management and list everything you see there.
If you have only ONE hard drive with three partitions then you should see Disk 0 with the three partitions. List how they are defined and the sizes, type and format type.
You may need to expand to full screen to properly view.