Well, in school, each class has 2 computer (desktops) for the student and teachers to use. 

And this time, 2 boys were fooling around,

and bump in to the computer (1 of them)

and, we can't start it up now

it can only go to choose how to open windows

if that happens to ur's

there is 3 pssibilities

1: the widows corrupted

it maybe caused by installing too much thing or turnning computer off not correctly

2:have virus  in ur computer

may cause by downloading too much things

3: power supply broken

the hardware power supply in ur computer might be broken, and cause ur computer keep rebooting

for 1 & 2, u can basically format the whole hard drive, and reinstall the windows

for 3... change the power supply! (buy a new one)