Vista inaccessible from other operating systems on multiboot system

Recently I have found that although Vista is visible from other OS's on my multi-boot setup it recently has been inaccessible. What could I try to make it accessible again. I recall some years ago following a guide to prevent system restore points from being deleted after booting into Windows7 and I made a fairly drastic change but I can't remember what it was. Up to a month ago I had full access to Vista from Windows 10 but now the Vista icon appears to be empty and is inaccessible. What could cause this?

 

One thing: Do this in Windows 10! Go to the Disk Manager -- there are multiple ways to get there, but one path in Windows 7 is to click on the Start Button, select System and Security, then Administrative Tools, then "Create and format disk partitions". Check to see if the drive with your Vista OS is visible, and whether a letter has been assigned to the partition. You might want to run a system check utility (Piriform's SPECCY would be fine) as a double check if your Vista drive seems to be missing. And if the drive is there, but not registering for some stupid reason like installing yet another operating system, then it's straightforward to add the Vista drive and assign a letter to the partition.

Second thing: if you've got Vista back on your system, at least as a partition that shows up when you click on Windows Explorer, you should rerun your multi-boot manager and make sure Vista is shown.

Zeroth thing: I've got Vista, Win 7, and Win 10 on my system (and I'm mumbling about which version of Linux I want) , and This Has Happened To Me. So, feel no embarrassment.