I wish I was smarter.

that LG 32UD59-B 32" 4K UHD display appears to be a 4K display with 10 bits/color, refresh rates 56 to 61 Hz (no 30 Hz).

connectivity includes:

2x HDMI 2.1o

DisplayPort 1.2

to run that display over DIRECT HDMI port, you must use special cables:

HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:

"PREMIUM High Speed HDMI cable" or that + "with Ethernet" (up to 4K at 30Hz) --OR--

“ULTRA High Speed HDMI cable" or that + "48G" (supports higher resolutions and backward-compatible)

Cables with No Certification tags are good for your standard 720p TV set, and not much more.

Cables 'shipped in the box' with many displays can be "lowest bidder" cables that are not up to high-end display use. Cables with no markings 'found' behind your TV set are unlikely to be good enough.

HDMI was invented for HD TV sets. it works great at its original resolution of 720i or 720p. At higher resolutions, it quickly develops issues that are complex to solve, and the cables and adapters required to solve are NOT intuitive.

"MacBook Pro 2021 Running Golden Gate 27.2 My monitor was working with my MBP through the HDMI port this morning, then it stopped for some reason. [...]Lmk if you have any suggestions. Because I'm all out of ideas."

-------

Excellent job troubleshooting this.

If a Monitor Does Not Work:

A. Single-out this user.

For Troubleshooting purposes, create a Time Machine Backup of your Mac (for restoration purposes), and then create an Administrator user. See if it then is accessible. If so, then it's an error with your current user. If not, then it is a system-wide error (perhaps the port).

B. Verify the Connection:

If you can access your Mac, verify the connection:

1. Hold Down: option button
2. Click: Apple menu
3. Select: System Information
4. Select: Graphics/Displays

C. Mirror the Screen:

Hold Down: [command + F1], seeing it your internal monitor is mirrored.

If you are already using PREMIUM HDMI cables:

at 10 bits/color:

• Your Mac can not produce a display at a data rate higher than 4K at 50 Hz. No through the Direct HDMI port, not through the USB-C port and HDMI adapter.

• But your display may not be able to do 4K at 10 bits/color at 50 Hz, it can only go as low as 56 Hz, if the specs are accurate.

To get a picture, you would have to reduce the display settings to 8 bits/color (sometimes shown as HDR=OFF) or lower the resolution.

... or use a USB-C to DisplayPort adapter, which CAN attain 4K at 60 Hz at 10 bits/color.

You could also run that display with a USB to DisplayPort adapter/cable , limited to ONE meter maximum length.

it could run at 4K 60 Hz 10 bits/color.

if using Display Stream Compression, it could even support higher refresh rates (but your display does not appear to support higher rates)..

<<. both will display my PC, but not my MBP. >>

That's not a meaningful comparison

The Apple standard for its built-in hardware-accelerated displays, makes them suitable for full-motion video for production/display of cinema-quality video. Folks really are producing commercial feature films on Apple equipment, so It MUST meet the highest standards. That can not tolerate any dropped frames, and NO dropouts or partial-blank scan lines due to memory under-runs or Transmit errors due to low-spec or overly long cables. 

These acceptable settings are obtained by a query to the display itself. In most recent versions of MacOS, no transmit errors are tolerated, or the display will drop out.

Under Windows, you manually choose the settings you prefer, or download a popular "Driver" (which is actually a package of resolutions and setting thought to be good ones). Whatever happens is your problem.

tjpowell84 wrote:

You are correct on that. Which is why I originally bought those specific cables. And they worked great for years, until they didn’t just the other day. I don’t think it’s the HDMI cables. The two I have are identical, and both will display my PC, but not my MBP.

With the release of macOS 26 Tahoe last year, I've seen a huge increase in the number of people complaining about issues with their external displays and external storage drives. When they replaced their older cables which had previously been working fine with shorter length & higher quality cables as @Grant has mentioned.....their external display & storage issues went away.

I consider @Grant the grand master contributor here on the forum when it comes to Apple computers and external displays. Their level of knowledge in this area is extensive. I have not seen any other current forum contributors come anywhere close to @Grant's knowledge & support in this specific area. If you search this forum, you will find @Grant assisting with the majority of these display related issues.

You came here for assistance because you tried everything else, please trust @Grant here. People should always eliminate the simple things first and this is one of those very simple things....using the proper high quality cables. None of the other troubleshooting steps are worthwhile until we are certain the cables are the proper ones.

Command-BrightnessDown toggles the sense of "Mirror Displays"

when you invoke mirroring, your computer calculates a compromise resolution that all display seem able to attain.

the built-in display at 3024 by 1964 is somewhat less than true 4K, so that resolution should be less demanding

it does NOT appear to drop the HDMI back to the next-lower speed step, and therefore is unlikely to change anything if cable spec if the issue.

That appears to be a Certified ULTRA rated cable. That cable is one step up in capability from the minimum your display requires, which is PREMIUM. These are backward-compatible to all slower speeds

Because HDMI uses much higher power levels than USB-C or DisplayPort, you will likely get away with TWO meters length, because you are not running as fast as a display cable Could run.

If you were buying a DisplayPort cable, the limit is ONE meter, maximum.

exactly what model Mac including processor M-___, {plain, PRO, MAX}

what make&model display?