Not knowing what version of Ubuntu you are currently running or your hardware I would recommend installing a later version of Ubuntu where the kernel is a later version and checking to see if it resolves your issue, otherwise you can follow this LONG Ubuntu Forums thread to try and solve the issue yourself.
This very well could be a Hardware issue. When your system doesn't boot what do you see on your screen does it hang? If so is it before or after the Ubuntu screen.Believe it or not answering these questions can tell us alot.If it hangs before the Ubuntu screen its Probably a hardware issue Does your screen stay blank. In which case could be a faulty battery or motherboard .
If its after the Ubuntu screen it could be a number of things causing this. Which we will need as much information as you can give. As in where its locking up and what your screen is showing and has showed during boot. Then maybe we can pin point what exactly has went wrong.