Microsoft has released official knowledge base articles for problems relating to your Windows Phone startup. So if your Nokia Lumia never gets past the Nokia logo or exclamation mark (for example), take a look at these knowledge base articles for possible solutions. The KB articles applies to all Windows Phones (Nokia, HTC, Samsung, LG, Huwaei, Fujitsu, ZTE, etc).
Windows Phone 8:
http://support.microsoft.com/kb/2749480
Windows Phone 7:
http://support.microsoft.com/kb/2606900
If Microsoft’s solutions won’t solve your problem, contact your device manufacturer’s service center (or place of purchase).