I connected a second monitor to my laptop with an HDMI cable but Windows doesn't detect it. The monitor works fine with other devices.
Since the monitor works with other devices, the cable and monitor are fine. The issue is on the Windows side. Press Win+P and select Extend — this forces Windows to look for a second display. If that does not work, go to Settings, System, Display, and scroll down to Multiple Displays, then click Detect. If still not found, your HDMI port may not support video output (some laptops have HDMI-in only on certain models), or your graphics driver needs updating. Open Device Manager, expand Display Adapters, right-click your graphics card, and Update Driver. For laptops with integrated Intel graphics and a dedicated NVIDIA or AMD GPU, make sure you update both drivers. If your laptop only has a USB-C or Thunderbolt port and you are using an adapter, make sure the adapter supports video (not all USB-C ports support DisplayPort Alt Mode). Also try a different cable — HDMI cables can look fine but have broken internal wires. If you are using a USB-to-HDMI adapter, it requires its own driver to be installed — check the adapter manufacturer's website.
