This discussion can be tricky because there are so many different mobile devices and proprietary methods for troubleshooting certain issues, however the common issues would be no power, blank displays, freezing or hanging up, memory issues, and more. many mobile devices need replacement components to resolve these issues such as removing the battery, replacing the memory, some mobile devices allow for the upgrading of physical storage while others do not. If it is a laptop, sometimes you can replace the keyboard or touchscreen should your troubleshooting for blank display lead you to replacing the display. Many instances it can be resolved simply by finding the upgrading the graphics driver or graphics card.