Despite Rockstar's best efforts there are still some outstanding issues with Grand Theft Auto Online, chiefly the fact that some players still appear to be losing their characters (if the reports we're getting are correct)!
However if you login to GTA Online and can't see your character all may not be lost. If you're lucky it might just be sync issues with the cloud and your avatar will still be stored on Rockstar's servers. You just need to check whether this is the case and here's how...
First, do not start a new character if you login and your previous character is no longer there. Instead, simply head to Rockstar's Social Club page (link), login and see if you're created character still exists there. If it does you are in luck, it means there are just some synchronization issues between Rockstar's servers and your console, and your character will eventually show up again when those issues are fixed. You just need to wait.
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If, on the other hand, there is no sign of your character on the Social Club then it's bad news we're afraid - it's highly likely they are gone for good and you'll need to start from scratch. It could be possible that Rockstar somehow figures out how to retrieve all the lost characters, but we don't think this is very likely. So you can wait for a fix that might never come or bite the bullet and start from scratch.