PPP

Have you tried rebooting the modem?

Next step would be to check all connections.

Tap the reset switch, or plug/unplug it.

If that doesn't solve: Take out the coaxial cable, wait a minute or two for all the lights but power to go out. Unplug power, wait about five minutes. (all times are rough estimates to cover many different makes. Mine takes like 30 secs each cycle)
Replug the coaxil cable, and after the power. Wait.

If problem persists, call up your ISP--especially if it's their modem, or you're renting it from them.

PPP is peer-to-peer protocol, the handshake between your modem and the DSLAM. The protocol in full will be over ATM (PPPoA). Basically if the connection has been dropped, the router should try to reconnect (when happy, most devices show it flashing multiple times after the adsl light goes solid); if it does this and then blinks out, staying off for longer than 2 minutes, most will recommend doing a hard reset. This means to push the rest button in for approx 20-30 seconds. If when released the adsl light flickers, goes solid and then you get a blinking PPP, great. If not, then you need to call your ISP to ensure the RADIUS protocol has been initiated: your router has a unique user name and password associated with your account stored in it. Sometimes this is dropped (your ISP can auto-connect and relog if you're on ADSL2 or better), or if traffic is high (or ports on DSLAM are restricted due to hot VP) the connection is passed into lockdown (one-way bridge) or state 24 as the network engineers call it. Your ISP can request the RADIUS platform be rebuilt on RAS and confirmed on SIDB which takes about 20mins to restore connection.