Check out Raspberry Pi's Troubleshooting Page for answers to questions such as:
What is the username and password for the Raspberry Pi?
Why does nothing happen when I type in my password?
Why does my Raspberry Pi not start up/boot?
Why is my Raspberry Pi hot?
I keep getting a lightning bolt symbol and messages about power...
My SD card seems to have stopped working.
I've imaged an SD card with Raspbian/NOOBS, but when I look at it with my Windows PC, it's not all there!