it used to work, but no more

If it works once, most electronic devices will continue to work almost indefinitely. There are a few exceptions:

• nearly every electronic component will be permanently damaged or destroyed, if power or other connections are connected to something with a voltage too large to handle, or unexpectedly reversed. Designers often put a "protection circuit" -- such as a crowbar circuit -- between connectors and sensitive components. But with adequate protection, most components will last indefinitely:;
  • transistors
  • integrated circuits
  • ceramic capacitors
  • printed circuit boards
• Some things, even when used normally, will eventually wear out and stop working:
  • primary batteries
  • secondary batteries
  • incandescent bulbs
  • LED degradation: every LED (including the infrared LED in each opto-isolator) gradually gets dimmer and dimmer.
  • aluminum electrolytic capacitors ("wet electrolytics").
  • relays

High temperature and high current usually accelerates wear-out. Sometimes a heat sink can extend the life of a component.