Mint 18.1 Cinnamon 64-bit v.3.2.7
Simon is me, the user
Total newbie, so I’ll admit to blundering around.

The problem: Update manager suddenly not working. No updates listed. Initially it started as a red cross.

Clicking on the update icon>information gave this:

09.08@17:15 ++ Launching mintUpdate
09.08@17:15 ++ Starting refresh
09.08@17:15 ++ Initial refresh will happen in 10 minutes, 0 hours and 0 days
09.08@17:15 -- Exception occurred in the refresh thread: <class 'subprocess.CalledProcessError'>

It then greyed out completely, after which update icon>information gave this:

09.08@17:15 ++ Launching mintUpdate
09.08@17:15 ++ Starting refresh
09.08@17:15 ++ Initial refresh will happen in 10 minutes, 0 hours and 0 days
09.08@17:15 -- Exception occurred in the refresh thread: <class 'subprocess.CalledProcessError'>
09.08@17:25 ++ MintUpdate is in tray mode, performing initial refresh
09.08@17:25 ++ Starting refresh (including refreshing the APT cache)
09.08@17:25 ++ Auto-refresh will happen in 0 minutes, 2 hours and 0 days
09.08@17:47 ++ Starting refresh (including refreshing the APT cache)

In Terminal:

$ mintupdate gives me:

$ Command '/usr/lib/linuxmint/mintUpdate/checkAPT.py --use-synaptic 48234513 2>/dev/null' returned non-zero exit status 139
-- Exception occurred in the refresh thread: <class 'subprocess.CalledProcessError'>

$ sudo apt-get update gives me:

[sudo] password for x: (I insert my password which I assume is rejected)

Sorry, try again.

$ apt-get update gives me:

W: chmod 0700 of directory /var/lib/apt/lists/partial failed - SetupAPTPartialDirectory (1: Operation not permitted)
E: Could not open lock file /var/lib/apt/lists/lock - open (13: Permission denied)
E: Unable to lock directory /var/lib/apt/lists/
W: Problem unlinking the file /var/cache/apt/pkgcache.bin - RemoveCaches (13: Permission denied)
W: Problem unlinking the file /var/cache/apt/srcpkgcache.bin - RemoveCaches (13: Permission denied)
E: Could not open lock file /var/lib/dpkg/lock - open (13: Permission denied)
E: Unable to lock the administration directory (/var/lib/dpkg/), are you root?
simon@simon-W25CSW ~ $

$ apt-mark showhold gives nothing at all.

You were right to try with apt in order to troubleshoot this problem, but you seem to have given sudo the wrong password. How many times did you try this? Running apt as yourself gives the expected list of errors because non-root users don't have access to the local apt databases.

What happens if you type sudo su and give your password? That should give you a root prompt from which you can run apt.

$ sudo so (password) gives me Sorry, try again.

I'm now guessing that either something has become corrupt or that there is a different password for admin that is now lost.

Either way, I've now turned off auto updates completely, and may simply install the latest version of Mint. Interestingly, with the updates turned off, the machine now runs at twice the speed.

It's "su" not "so". But yes, you do seem to be trapped in a loop. One way around this would be to boot from your installation CD and use it as a rescue disc. That's certainly what I would do in your place. But you might not feel comfortable doing something so hands-on .

I haven't used Mint for a long time. It's based on Ubuntu so I assume that it has no separate root password. On Ubuntu-like systems, the first user has full sudo rights and their password is the only one that matters. Can you remember if you were asked for a separate root password when you installed the system?

Don't just turn off updates, at least not permanently! Out-of-date software is a security risk. It would be better in that case to reinstall, although with Linux we try to avoid that where possible. As I've said, most Linux problems can be fixed from a suitable rescue disc, but that's tough going for a complete novice.

I do not like autoupdates on principle. That's the Microsoft way of doing things. Running an update by hand once a month with apt or synaptic is much safer.

The problem might be that you are using version 18.1 and 18 is currently at 18.3. Since 18 is only supported until next April, you might be better starting again with version 20.

There is no admin password with Mint, so sudo su should work. It can't be that you've forgotten the password, because you used it to log in. Can you access synaptic and use that to update?

I'm beginning to wonder if the update screwed up his sudoers file. That would create a fatal loop, wouldn't it?

Occam's Razor would suggest that he is entering the wrong password. Perhaps the caps lock is engaged on the keyboard. Perhaps he's misremembering or mistyping it.

Run the command "groups" to list all the groups your user account is in, if you don't see sudo listed then you are not in the group.

Boot into recovery mode from grub, which should drop you into a root shell were you can add yourself to the sudo group by editing /etc/group and adding your user account to the sudo line.

Thanks to all for replies. I've decided to wipe the machine and install the latest version.