The operating system on your microSD card can sometimes be corrupted if the Raspberry Pi is not powered down correctly. See this excellent set of instructions from the Raspberry Pi Foundation for two methods to re-load Raspbian onto your microSD card from another computer with a card reader.
For reference, the best way to power down the Raspberry Pi when running Raspbian is;
- Onscreen, click the Raspberry logo from the menu bar, and select 'Shut down'
- Confirm 'Shut down' from the three choices
- Wait about 10 seconds or until the green activity LED on the Pi board stops flashing
- Unplug or switch off the power supply to the Pi board