Quick answer: The driver didn't read your monitor's EDID, so Windows fell back to a safe mode resolution — a clean reinstall with DDU plus a DisplayPort cable reseat fixes it in 90% of cases.
What's actually happening here is a timing failure during the EDID handshake. When your GPU driver loads, it queries the monitor over the DDC channel (pins 12 and 15 on HDMI, or the AUX channel on DisplayPort) to get the Extended Display Identification Data — the little EEPROM blob that says "I'm a 2560x1440 panel, 165Hz, here are my supported modes." If that query times out or returns garbage, the driver does the safe thing: it drops to a VESA fallback mode like 1024x768 at 60Hz. You'll see this most often right after a driver update because the new driver reinitializes the display subsystem and sometimes races the monitor's power-on sequence, especially if the monitor was in deep sleep when you rebooted.
This is not the same as the monitor being broken. It's not a cable that's suddenly bad. It's the driver giving up on the negotiation and picking a default it knows will at least display something.
Fix steps, in order
Full power cycle, monitor first. Shut down Windows. Unplug the monitor's power brick (not just the button — pull the barrel jack or the IEC cord). Hold the monitor power button for 10 seconds to drain the caps. Wait 30 seconds. Plug power back in, turn the monitor on, wait for it to show the input source, then boot the PC. The order matters because the GPU needs the EDID ready before POST finishes enumerating displays.
Reseat the cable at both ends. DisplayPort especially — the connector has a latch and it's easy to think it's seated when it isn't. Try a different port on the GPU too. If you're on HDMI and the monitor has a DP input, switch to DP. HDMI 2.0 cables go bad in ways that pass video but corrupt the DDC line.
Check Display Settings for the native mode. Right-click desktop → Display settings → Advanced display. If your native resolution isn't in the dropdown at all, the driver truly didn't read the EDID. If it's there but grayed out, it's a bandwidth negotiation problem (usually the cable).
Clean-install the GPU driver with DDU. Download Display Driver Uninstaller from Wagnardsoft, boot into Safe Mode, run DDU, select your vendor (NVIDIA/AMD/Intel), and hit "Clean and restart." Then install the Studio driver (NVIDIA) or Pro driver (AMD) rather than the Game Ready/Adrenalin build — the studio drivers are validated against more display configs and skip some of the aggressive mode-switching logic.
Roll back one driver version. If the problem started with a specific release — say NVIDIA 551.xx on a DisplayPort 1.4 monitor with DSC — go back one major version. Every few months a driver ships with a regression in the DP link training code. The 545.xx branch was rock solid for a lot of 4K/144Hz panels where 551 broke things.
If that doesn't work
Create a custom resolution. NVIDIA: open NVIDIA Control Panel → Change resolution → Customize → Create Custom Resolution, and punch in your native mode manually (e.g. 2560x1440 @ 144Hz, CVT-RB timing). AMD: Adrenalin → Display → Custom Resolutions. This bypasses the EDID read entirely and tells the driver "trust me, this works."
Override the EDID in the registry. Extract your monitor's real EDID (from the manufacturer's site, or from another machine where it works) and inject it. On Windows this lives under:
HKLM\SYSTEM\CurrentControlSet\Enum\DISPLAY\<MONITOR_ID>\Device Parameters\EDIDBack up the key first. Replace the binary blob, reboot. This is what a service tech does when a monitor's own EEPROM has gone flaky.
Use an EDID emulator dongle. A $40 device like a Dr. HDMI or EVGA Precision sits between the GPU and monitor and passes a known-good EDID. Sounds like overkill until you've spent three hours on a KVM that keeps dropping the DDC line.
If none of this works and the monitor shows the correct resolution when plugged into a laptop, the problem is your GPU's display controller, not the monitor or the driver.
Prevention
Don't update GPU drivers the same day they release. Wait a week — the first hotfix almost always lands within 72 hours and it's usually a display fix. Also, if you're on DisplayPort and your monitor has a "DP 1.4 / 1.2" toggle in the OSD, set it explicitly rather than leaving it on Auto. Auto mode is the single biggest source of EDID handshake failures I see on 1440p+ high-refresh panels. And keep one known-good cable taped to the back of your desk. You'll need it at some point.