When I start Minecraft I get the mojang thing then the screen turns black! help!?

either you have no room on your computer or theres an egsisting mod that has'nt been updated, if you want to update it just go back where u found it open your roming (on a PC u can press the flag thing on the bottom of the keyboard and press the R key simontaniusly, then press on the .minecraft folder and take out and thing u want then delete it, run minecraft and a new one will apper