This is a very old monitor. It's more surprising that it was recently working than that it is not. I can share some information on it but, unfortunately, don't have a good solution.
The connector is a bit of a puzzle. Those two missing pins served a purpose, but not for your monitor. This was an early form of "plug and play", used from the mid-1990s until 1999, called Display Data Channel or DDC. Those two pins, when not connected, indicated a monochrome monitor with resolution less than 1024×768. Your monitor is color, with a native resolution of 1280 x 1024. So the missing pins don't make sense unless the DDC protocol is ignored if the next generation (E-DDC) protocol is present (and that I don't know). The odds seem small that two random pins would break and coincidentally be those two.
Given the tests you've already done, there is really no way to know what's wrong without opening it up. It might be possible to fix whatever is wrong with it but realistically, it is way past its expected service life. Even if you fix it, something else is likely to fail. If you are a technician, have the tools and can find parts, you could tinker with it for the potential satisfaction of getting it working again. But to take it to a shop will likely cost you more than an equivalent new monitor, and a new monitor will come with a warranty. Even to have a shop open it up and diagnose the problem to give you an estimate would cost a major fraction of a replacement monitor. So it may be time to let it go to monitor heaven.