One of the most common solutions is to perform a hard reset (Settings > About > Reset your phone) - that would basically bring your phone back to near-factory defaults, erasing all your custom files, settings, etc.
The other solution is to reinstall the OS using the Lumia Software Recovery Tool, but it can become a bit technical along the way. If you're not comfortable with technical details/procedures, you may want to just bring your phone to a service center, bring your receipt with you, claim the 12-month warranty of your phone, and have it checked/repaired there.