What is wrong with my car 2003 nissan altima?

Yes i did...then changed the radiator...now the car doesnt start...cant hear the fuel pump turn on and car doesnt wanna start. Please help me

Also explain to me “Serpentine belt fell off” I’m curious as to how this happened or if you meant the belt broke

I know you said you tried to jumpstart it but letting the battery charge may help.. also check your belt route make sure it’s right because the alternator is charged with the belt