K850i says no network coverage constantly.

did you hit your phone?
try making 'Master Reset' (settings->master reset).if this doesn't work try flashing phone's firmware

how can get net work coverage in k800i sony ericsison mobeile phone

your phone must be an imported one if not contact your mobile vendor and have an unlock

Sounds like the phone hasn't been unlocked properly. I had a similar problem and eventually just changed my sim to the original phones network. This solved all problems but i don't know wether this is the full solution to your issue. I tried to help best as though dude.
Cheers.

Sounds like your phone is stuck in flight mode for some reason. Have you tried a Master Reset (you will lose all your personal data if you do this). 

Hi there. it could be one of a few problems, firstly, check if you have a 3G phone? secondly, check if you have 3G coverage? Thirdly, do you have a 3G sim card? If this doesn't answer your question, you should go to a service provider shop so that they can check it for you!

throw it out the validity might have gone

check your phone settings. tnx

well... seem like you phone has got locked from the network may be accidently in case of stolen better check you service provider and if they say every thing is fine try to flash your mobile .it has some perticular number to flash it try these number *#7780# (password 12345)