I had something similar happen after I increased the partition .
This is on a quadruple boot (multi boot drive) - Partition letter changed from L to K.
The way I solved this, was to use a bootable CD,DVD or USB drive with a partitioning program,
so I could work outside Windows.
In the program I removed the drive letters from the changed partitions and afterwards restored the drive letter to the "L' partition
Took out the bootable media and rebooted - For me this worked like a charm.
If you have a less complicated setup, you may well do this in 2 passes : 1 ,remove drive letter on the X drive.
Let it complete - Then reselect that same partition and now choose a drive letter again, C should be avaiable..
Let this also complete . When finished, reboot..
Hope this helps..