after a system restore and it comes back, run a checkdisk on it. in
Computer, right click on the drive, choose properties, choose the Tools
tab, click the Check Now button and allow it to be scheduled for next
time you boot (if that's what it asks).

Otherwise, while it's disappeared, do you see it in Disk Manager? Check
this by right clicking on Computer, then choosing Manage, then in the
box on the left click on Disk Management. See if your drive appears
there, but not in Computer. If it does, I still recommend running
checkdisk on it and making sure it is initialized. Right clicking on the
disk brings up various options.