Sony ericsson k800i

we will approach the problem this way
1) your battery might be the culprit , first charge your battery in a different piece, and then replace ans see.
2) if the above doesnt solve the problem clean the connectors of the battery that shiny golden part of both your battery and mobile piece , with nail p[olish remover or alcohol.
3) last resort now will have to open the piece and check for the on/off switch .
4)lastly software reflashing, but if the piece aint detecting, need to clean the datacable connectors with nail polish remover or alcohol.

the above should bring your pice to life.