more than likely either a power cut or you went on holiday
your router has probably assigned you a new IP Address since you last configured it (normally due to a reset or long time offline)
do you remember setting it up before? if you check your computers IP Address again then you'll probably find it has changed, so if you go back in to the router again and change where you forwarded to then it should sort it out
if you can't remember what you did last time or have problems with doing it, post the make/model of the router and we can help more
