Hi there. Sometimes it is a 3rd party app that is in the way. You can test by going into safe mode, a situation where all 3rd party apps are excluded in the startup process and you're running a more or less clean machine. Goto safe mode by pressing the power button. Then long press on the restart button and you get a choice to go into safe mode. Anather test is to run the phone without SD card. SD cards tend to break down every once in a while. By taking it out you can test if the SD card is causing trouble. Just ask again if you are still having problems.
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