DIAGNOSIS The fan motor is getting power, but the control for the lower speeds has failed. This is usually a problem with the circuit board behind the control panel, specifically the triac (a small electronic component) that regulates speed.
FIX You’ll need to replace the main control board. 1) Unplug the fan. 2) Remove the control panel cover. It’s usually held by a couple of screws and snaps at the top. 3) You’ll see the green circuit board. Take a clear photo of the wire connections before you disconnect anything. 4) Disconnect the wires, remove the old board, and install the new one, reconnecting each wire to the exact same spot. Search online for "Lasko 2511 control board" to find the right part.
IF THAT DIDN'T WORK Less likely, but possible, is a fault in the wiring harness between the control panel and the motor. Check all the wire connectors for corrosion or looseness while you have it open.
HEADS UP When handling the circuit board, avoid static electricity. Touch a metal part of the fan’s frame first to ground yourself. Double-check your wire connections against your photo—a wrong connection can cause more damage.