I bought a PRS CU24 in 2004 and never had to set it up again. Because I left to live abroad, I had to leave it in its case for 3 years without playing. When I came back I tried to play on it but it was really buzzing. So I removed all the strings, cleaned the neck and the frets with the fretboard conditioner and a cleaning cloth. I then restringed the guitar with my usual gauge (the stock one ie. PRS 9.5-44) but it's still buzzing like hell on almost every frets!!! Open strings are not buzzing for some reason.

What can I do to stop it from buzzing?