iPod touch iPod Touch error message ? - help needed

Whenever i connect my ipod to my computer it does all the usually automatic sync but it has somehow taken all my music, pictures and videos off and won't put them back on again.
I purchased the ipod update and that was sucessful but when i unplugged my ipod and went to listen to music i found there was nothing left on it.
So i went onto my itunes and it appeared with a message saying "unknown error occured, could not complete sync"
Then i brought some music straight off of itunes on my ipod and plugged it in again and once again it took the music off. I havent lost the music, it is still on my itunes database, i just don't know what to do to get my music back onto my ipod.
Please, i need some help or advice on what to do as this has confused most people i ask. thanks

Ok i havent done that yet but i shall try that and see if that works, what does the backup do and why should i not do that? thanks

ok, right a restore it is for now and let's see how that goes! i'll tell you how it goes, thanks alot, let's hope it solves my problem, cheers

It's half worked!!

It's half worked!!!!! It took quite a few hours but i have managed to get a few songs back on my ipod but it still doesnt upload all my songs and an error message still appears?! But atleast i have some songs on my iPod for now! thank you

Still not right!

I have done all the troubleshooting but it still isnt right! It still keeps coming up with 'unknown error occured, ipod could not be synced'! I've checked all the songs are selected and that it is authorised to my computer.
It wont allow me to put anymore songs on here and i recently brought some songs on my ipod, plugged it in, it started to sync and after it had synced i now cant listen to those songs anymore as it flickers and than goes back to the list of songs!
I dont know what is going on as all i want is for me to be able to put my new cd's onto my ipod but yet it wont sync properly and keeps messing up my music! any idea's would be grateful ?!
thanks