You connect a probe but can't talk to the target. What do you check?

Several common causes, roughly in order. First, the OpenOCD (or vendor) configuration: you need the correct interface file matching your actual probe (ST-Link vs J-Link vs CMSIS-DAP) and the correct target file for your exact MCU; a wrong or missing config gives "no target found." Second, the physical/electrical connection: the probe usually senses the target's logic voltage via the VTref pin rather than powering the board, so you must power the target separately, share a common ground between probe and target, and connect VTref so the probe drives the SWD lines at the right voltage, missing VTref or GND is a leading "won't connect." Check the SWDIO/SWCLK wiring and the reset line too. Third, firmware locking out debug: if the application reconfigures the SWD pins as GPIO, enters a low-power mode that gates the debug clock early, or readout protection (RDP) is enabled, the debugger can't attach to a running target, the fix is often "connect under reset," which halts the core at reset before the firmware disables debug, or lowering the adapter clock speed if the connection is flaky due to wiring. Fourth, adapter speed too high for the wiring causes intermittent failures, so reduce it. And confirm the probe itself is recognized by the host (drivers/permissions). The systematic approach is: verify config (probe + chip), verify power/VTref/GND/wiring, try reset-and-halt / connect-under-reset, and lower the clock, most failures are config or VTref/ground.