I bought a Nokia 5800 about 6 months ago of ebay, it worked fine then today all of a sudden I can't get any signal at all, I have tried sims from other netwrks again no signal, What is going on?

You tried turning it off then back on?

It's "possible" that it's been blacklisted - when you make a call do you hear 3 short beeps?

Hard not to when trying different SIMs....

Yeah I do

Sounds like it's been blacklisted then - this happens if it's been reported lost or stolen by the original owner. If that's the case and they're claiming on their insurance then they're committing fraud. It's up to you but the bad news is if it is blacklisted then you're left with a 'bricked' phone.

This has happened to me in the past, I've gone to the police about it. Might be an idea to contact the seller to let them know the situation.

It's worth ringing your network just to clarify whether it has actually been blacklisted or not, it could've possibly just developed a fault.. It does however, as iRuss said, sound blacklisted.

I made a mistake. Apologies. Now string me up and stone my ass.

It's a pretty common occurrence wrt mobile phones on ebay these days.

The sellers claim the phones from their insurance companies claiming they've been stolen so they end up with double the money too.