I have a hAP ac2 (currently on 7.1.2) that's been running fine for the last 2.5 months. Today, while working on a wired connection next to it, the lights on the ac2 went out. I tried unplugging and replugging it. While the lights show activity on the wired ports, nothing connects. I finally reset by unplugging the power, holding the reset button, and replugging it and waiting for the usr light to blink. It reset to the default 192.168.88.1. I can ping that IP, but trying to connect to the web interface fails and I get connection refused. I tried downloading winbox, but connecting via IP or MAC also get connection refused. I can't find any way to get in to see what's going on (newbie here anyway). The working setup was simple - 2 wired machines, 1 wired AP and wifi. Any suggestions or this now just a brick?

I also tried etherboot on the first port, but couldn't get netinstall to see the ac2, following the miktrotik instructions.

You have a couple of options.
First try again with the MAC address of that device but use the ethernet port number 2 (the one next to wan if it has one), since this port is most likely able to accept winbox connections
If that fails then you should try using the windows tool called "netinstall". Keep in mind that you need a netinstall that has the same version as the image you are trying to send, you will also need the image for your device - you can get all of those things in the software / downloads section of the mikrotik website.
If your pc has no firewall on (and no antivirus firewall) you should see your device there when it boots, and you can send your image to it to reflash it.

Sometimes you need to press and hold the reset button a few seconds so the device enters netinstall mode.

^ please note all of those comments were done from the top of my head and are, by no means, a precise step by step guide. All the comments stated above should be seen as suggestions and tips to point you in the right direction.

Thanks, kryztoval. I've tried all those variants. While it responds to ping on the default 192.168,88.1, winboot with either IP or MAC on either port 1 or 2 gets "connection refused". As for netinstall, I followed the directions in https://wiki.mikrotik.com/wiki/Manual:Netinstall, but I can't get it into etherboot such that netinstall shows the device. I've also tried via a different NIC.

Today, while working on a wired connection next to it, the lights on the ac2 went out.

Seem likely that this wasn't just a coincidence. What were you working on?

Have you tried connecting something else to the port the ac2 was plugged into?

BTW, the ac2 failed on its own, not while I was doing any upgrades or config changes. I was working on something else and just noticed the lights were out, except for the power light on the back. It had been working with no problems up to that point.

Today, while working on a wired connection next to it, the lights on the ac2 went out.

Seem likely that this wasn't just a coincidence. What were you working on?

Have you tried connecting something else to the port the ac2 was plugged into?

Sorry I was unclear - I meant that I was sitting next to the ac2, working on a desktop machine connected via a wired connection (not wifi), but I was working on a spreadsheet, not anything related to the router or even using the network.

The ac2 was plugged into a cable modem. When it died, I replaced it with the hEX RB750Gr3 and separate AP I was using previously - that's all working ok, so the port on the cable modem is ok.