Blue screen at startup

The most likely cause of this behavior is a third-party System Preference that has become too old to be compatible with the version of OS X you are running.
Boot the machine and then immediately hold the shift key down for a "safe boot" until the login screen appears. This should get you to the login screen. From this point, you can use the Console utility to review the logs from the times of the last boot failures and see what program is complaining about failure and why. Or if this is too technical for you, you can open up System Preferences, identify all your third party extensions (anything appearing under "Other"), download the very latest versions of each of them and reinstall. That has a better than even chance of solving your problem.
Keep in mind that even if it doesn't, your problem IS solvable by somebody who can read your logs, you are at no risk of data loss, and your machine in all probability has no hardware problems. There are also other techniques to resolve boot-up problems if this one is unproductive, just come back and ask.