First, let me say I'm not really super tech-savvy. more of a casual tech-tourist, but I'm learning fast and trying my best, so please bear with me

I'm admin on a forum that started disappearing members, myself included. i read the long sticky about this, but it's pretty old and we're not on heart. I don't even know what heart is.
The strange thing is that I can't access my username on my computer, or any other computer, but my partner's computer logs me in no problem. At least I can still get into the site and access the admin panel!
Several members have been "locked out", some have created new usernames to keep posting. one member created a new username, but then got locked out of that one, only to find that she could log into her old username no problem, but not the new one.
a couple of posts seem to have disappeared, but only one or 2 that i know of; doesn't really seem to be a big problem - just usernames disappearing. Some of them have disappeared from the user list, but others are still there, and appear normal, yet the member can not log in. It says "invalid username".
We do have some backups but i really don't know from how long ago (another admin was doing that, but she's away for the now). I have no idea how to restore the forum from the backup either. I would like to know if i do manage to restor from the back-up, will that return the disappeared users, or will we still have the problem, and if i make a new back-up now, will that mean that all our forum is saved, but without the missing users, and never to be returned?
i'd really appreciate some guidance here, thanks in advance

Try rebuilding your members list.  That may fix it.

thanks justin. i see the function "rebuild members list" in the admin control panel.
if I click on that, what happens? do i need to manually type in every user?

ETA: I clicked on it and it said "are you sure you want to do this?" which got me scared that it's some kind of big deal. then I clicked on another "rebuild members list" button in the "forum quick links" section anyway, just coz I'm feeling reckless and desperate. it seemed to do something, and a little message popped up saying it had been done. so I'll see if that fixes things.

Just answer "yes" to that "are you sure you want to do this" prompt.

It isn't really a big deal, Just will put your forum into maintenance mode while its being done, Unless you have hundreds or thousands of members, It will take simply seconds to complete, So nothing to worry about.

thanks. it did just take a few seconds; we have about 150 members.