If you have a recovery drive, you should see a message on the first screen to show up when you power up the machine as to which key to press to go into it. However, it will take the system back to the day it was installed so you'll lose all your data and settings.
There may also be a Recovery icon in the Control Panel.
If you don't want to do that, tell us what the problem is and someone will be able to guide you through the better way of fixing it.