Hi revaN and welcome to the forum!
Please follow the steps outlined in this tutorial - Hard Disk Partition Letter Missing in Windows 7 - Fix
If you still encounter the same errors, please try to download and use TestDisk, here
Be sure to read the software documentation carefully before proceeding with running the software. Alternatively
, please try booting off of your Vista partition and do a check disk on the drive in question. Check to see if the issue is still present when you boot back to Windows 7 then try the suggested steps above.
Let us know how it turns out