big problem upgrading from 10.04 to 10.10

i upgraded from 10.04 to 10.10 from the update part of ubuntu. after it downloaded the necessary files and installed them it had to restart. after restarting Ubuntu will not open up. my computer will turn on the the boot screen where i can choose from vista or ubuntu. i choose ubuntu and it comes up the no wubildr message then the next screen says error: no suitable mode found. Someone please help! i dont know what to do!