Same problem here. Sometimes drives disappear completely from the Finder sidebar. Sometimes they just gray out. Quick 'n' easy fix: In Terminal, type:
chflags nohidden /Volumes/MY_PARTITION
(where MY_PARTITION is the name of the drive you can't find).
For the UNIX impaired, if there's a space in your drive name, you can use "\ " to represent the space (no quotes, just the backslash and the space). So for example, Macintosh HD would be typed: