Just re-enter your username and password or re-login if you are logged out.I believe if you are visiting Pprune as a guest there is a limit to the number of threads you are permitted to view. Every now and then the servers at work get reset and all your history files and cookies are deleted,this will also happen if there are any outages or power cuts.
A MOD/BT message will appear if you try to access a website that is not permitted ie,Ebay and as far as I'm aware Pprune is not a restricted site at work,YET!