iphone turned off

my iphone recently turned off unexpectedly and wont turn on anymore. when it does try to power on again it only takes me to a screen that says slide to upgrade & when i do it just takes me to a blank page & back to the original page (slide to upgrade). I would like to know what i can do & how i can fix it or get it fixed. it doesnt do anything other than that. I have tried to restart it & even tried to reboot it & reset it but it doesnt let me do anything. Please help.