If that’s how the company handles this, I would start to look for a new job immediately.
- no indication in the post that they going to hire a new person
- If your company IT infra is down, you are probably dead in the water, leaving it to be managed by a single person who does not have any IT infra management experience is a pretty bad business decision
- No indication in the post that the person offered any training or anything
- my experience with 1 person responsible for IT support/management/sysadmin jobs are that they are 24/7, which leads to burnout and mental health issues
I enjoy sysadmin, partly because of the variety of problems you are called-on to solve. But the fact is, if you do the job well, and everything works well, nobody gives you any credit. When things go wrong everybody will be looking at you, and if you haven't fixed it within 20 minutes, then you must be incompetent.
So you're the only sysadmin in a multi-campus company? As someone noted up-thread, you can't do it alone. At least, you need someone on each site that can act as remote hands for you, even if that's not their main job. And you need people to delegate grunt work to - repetitive stuff, like building workstations for new hires. That's IT, but it's not sysadmin.
Get some interesting hardware to take home and play with. Stuff like smart switches and firewall boxes (the same gear your employer uses).
GNU/Linux has much better networking tools than Windows; if you're in a Windows shop, get a laptop with Linux on it.
Whenever you do a manual job you've never done before, let part of the job be automating it, so you never have to do it by hand again. Let the automation scripts be the documentation.
Learn to be pessimistic (a common sysadmin trait). If you have bad news, don't ever sugar-coat it. Things might not be as bad as you thought, in which case your reputation is improved. But what people don't want to hear is that the job takes two hours; and then find after two hours that it's a two-day job.
If you can get yourself a full-time assistant/colleague, work as a pair when you can. It reduces stress a lot if you have someone checking your work as you do it (and your decisions). And you end up with two people knowing the systems, not just one.
Get a hardback A4 notebook, and record everything you do.
In theory, you have a lot of power (c.f. BOFH).
You know stuff others don't, and you have superuser powers everywhere. Don't abuse it. Be as helpful as you can. People will be nice to you if they know they might need your help one day. The BOFH was a tyrant, and I don't think he'd get far in a modern business (the BOFH worked in a university IT department, and was God; a sysadmin in a modern business can't behave like the BOFH).*
And seconding the physical logbook.
And it's easier to scribble a line in a physical logbook than to crank-up some editor, open the logfile, append an entry, save the file etc. If you're going to log absolutely everything, then adding an entry needs to be as easy as signing your name.
You could rely on your laptop; but laptops break and crash. a physical notebook is crashproof, drop-proof, and can even survive being dropped in a puddle. And short of pencils and erasers, it's intrinsically an append-only medium (except that you can still annotate old entries).
Maybe an electronic logbook works for you; I found that my logs were more comprehensive and generally easier to use if I kept them on paper. I got through about two notebooks a year; I used a stack of old ones to elevate my monitor (phone directories are hard to come by these days).
Also, opening a big notebook before describing your activities confers an aura of reliability on you; there's a reason why policemen whip out notebooks when testifying in court.