Let us know what (if any) troubleshooting was done. When it won't start / run, is it getting the three critical conditions required TO run?
- Compression
- Fuel
- Properly timed spark
Report back and let us know which is missing. If you have all three, it will run. So one of the three is missing. One is very easy to test, one easy with a timing light - but if you have no timing light, you can at least verify spark in general without one - and the other would require a compression test tool.
It could be anything... Vapor lock, valves too tight that heat up and cause you to lose compression, a failing ignition system (such as the pesky electronic ignition box going out), etc.