If a switch fails to turn on or if its toggle feels loose, replace it. A single-pole switch is the most common type of switch. It has two terminals (not counting the ground), and its toggle is marked with ON and OFF. If three wires attach to a switch (not counting the ground), it's a three-way switch. Replace a single-pole switch with an exact match or install a dimmer or special switch.
About 20 minutes to replace a single-pole switch
Screwdriver, strippers, side cutters, long-nose pliers, voltage tester
Stripping wires and fastening them to a terminal
Lay a towel or small drop cloth on the surface below the switch