When the replacement cost of a slow cooker is less than the repair cost, it is time to buy a new one. The most expensive repairs generally involve the heating element or controls for the cooker. If the crock has cracked, replacement of the whole unit may cost less.
Slow cookers either experience failure to heat, heat food too much or develop leaks. If the cooker doesn't heat, check the power in the outlet and the electrical cord. Use the owner's manual to find the process for testing the control dials and the heating element. In cases of excess heat, the thermostat may need replacement. All of these repairs are likely more expensive than buying a new slow cooker, though.