There are many basic details lacking, that could be helpful in determining the cause of
whatever issues are behind symptoms you've stated. Some of the first items to include
would be a more detailed description of the MacBook (exact model, specifications, etc)
and the version of OS X running in the computer.
• Information about my Mac - Apple Support
Information can be found in several places in the running computer, or ahead of trouble
to visit those informative sections & write down the product specifications for offline use.
You could try starting up with the Shift key held down, in "safe mode" to help troubleshoot.
• Try safe mode if your Mac doesn't finish starting up - Apple Support
{safe boot mode attempts a repair on startup similar to...}
• Repair disk permissions with Disk Utility - Apple Support
The instructions may allow the computer to start up, but the system would be in limited form.
• About the screens you see when your Mac starts up - Apple Support
At this time, what computer are you using online to post here and also to perhaps search
for answers to the question or to learn about how to handle care & feeding of Mac OS X?
Not sure what else to suggest now, however if you aren't far away from an official Apple
Store (retail, not a reseller) you could get an appointment with the Genius to have some
one there look over your computer(s) and offer suggestions as to further action to take...
Anyway, hopefully you can document what you do with your MacBooks, and state their system
configuration information, which exact computer and so on, in a reply. Perhaps someone else
may be of more help. I suggest the Genius at an Apple Store; contact them for reservation and
take both (if you still have them) ailing MacBook computers. They likely suffered a similar fate.
Good luck & happy computing!