Hard to tell exactly what happened without a screenshot from Disk Management, but if the partition is still showing there, chances are your files are fine, and Windows just lost the drive letter for that volume. Seen this happen pretty often after people poke around in Disk Management.
First thing I'd check is what the partition actually looks like in Disk Management:
If it still shows as a healthy NTFS/exFAT partition with a blue bar, that's usually a good sign.
If it says RAW, the filesystem may be corrupted.
If the space shows as Unallocated (black bar), the partition itself was probably deleted.
If it's the first case (healthy partition but no D: ), try this:
Right-click the partition
Choose Change Drive Letter and Paths
Click Add
Pick an unused drive letter (doesn't have to be D: )
Apply it
A lot of the time the drive pops right back up in File Explorer immediately.
If Disk Management won't let you assign a letter, you can try DiskPart instead:
diskpart
list volume
select volume X
assign letter=D
exit
(Replace X with the correct volume number from the list.)
One thing to watch for: make sure D: isn't already being used by something else like an optical drive or mapped network drive.
If the partition now shows as Unallocated, I'd stop using that drive for the moment. Don't create a new partition or format it yet. Depending on whether it's an HDD or SSD, and whether anything has been written to it since, recovery may still be possible.
Also, if you can, post a screenshot of Disk Management. That'll usually tell people here within 10 seconds whether this is just a missing drive letter or an actual partition problem.
