In that case, it is going to be a process of elimination.
Is your PC otherwise working properly? If not, unexplained strange behaviour can sometimes be down to a virus.
If it is otherwise ok, have you recently changed you ISP, upgraded or changed your browser software or installed any other internet related software?
If none of these apply, then you may need to get your modem tested as it could have developed a fault (does happen), or get a cheap software modem to try.