If your service was working and now isn't, and you've changed nothing, chances are that the network may be experiencing an outage and that the best thing to do is change nothing and just wait a while for the problem to be solved. Usually network problems will be solved within an hour. Try power-cycling your modem.
If your service is chronically sub-optimal, below are some possible problems and possible solutions.
Background information about DSL
The low-frequency end of your phone line is reserved for voice signals. DSL works by using higher frequencies. Line filters keep the DSL signals from affecting non-DSL devices, and also keep those devices from 'contaminanting' the DSL portion of the spectrum.
When your modem detects the DSL signal, it tries to synchronize with it (go 'in sync'). If it can do this, the modem is said to be 'in sync' and the DSL/Broadband light will be green.
When a DSL signal is detected, the modem assesses the power spectrum of the line and decides how much bandwidth is available. If the line quality is poor, the modem will operate a low speeds to reduce errors. For best performance, good line quality is essential.
It is Bell Canada's responsibility to ensure that DSL signals reach your modem with sufficient strength and quality to allow high speed internet. You have the right to cal their service department at 611. However, they sometimes will conclude 'sorry, that's the best that we can offer at this time' or will charge you if it turns out the problem is within your residence. So it's good to check what you can before calling Bell.
Generally the approach is to try the best possible arrangement for your modem (no other devies on your phone line, and the modem as close as possible to where your phone line comes into your house, away from all sources of electrical interference) and seeing if that improves quality. If that works, then something about your normal configuration is a problem. Otherwise, maybe it's an external (Bell) problem.
'No sync', 'intermittent sync', 'poor performance', and 'low speed'
Here are some possible causes of 'no sync', 'intermittent sync', 'poor performance', and 'low speed':
DSL modem is connected to the phone line via a line filter
The fix: Remove the line filter. The DSL modem must be directly connected to the phone line, without a line filter.
Modem not connected to phone jack.
The fix: Connect the modem to phone jack at which there is dial tone.
Telephone(s) or other devices not filtered on the same phone line.
The fix: All devices (other than the DSL modem itself) must be connected via a line filter (otherwise they may interfere with the DSL signal). Install filters or disconnect the other devices from the phone line.
Outside line issues: Noise on your phone line. If you hear cracking, static, etc on your phone line, this can interfere with your DSL signal too.
The fix: Call 611 to report the problem to Bell Canada. Tell them about the voice quality on your line being poor due to cracks & pops & static sounds.
Defective filter(s). If the DSL modem works well with all other devices temporarily unplugged from your line, maybe one of those devices has a defective filter.
The fix: Try to figure out which filter is defective by plugging the devices in one at a time. If one is found to be defective, replace it.
Faulty inside wiring / phone jack(s). If your problems appear after adding a new phone or other device, or changing the wiring in your house, those changes are likely the cause of problems.
Try to repair your inside wiring. If unsuccessful, Bell Canada offers a charge-by-the-hour service to find problems within your residence -- call 611 to arrange for assistance.
Modem connected to wrong line (multi line household).
Distance from DSL service exceeds maximum distance. If you've never been able to obtain sync, maybe your location is too far from the nearest Bell service point. NCF can work with Bell to assess this; contact NCF via the Help page.
Electrical interference from AM radio, halogen lights, audio speakers, power supplies.
The fix: Location the modem away from sources of electrical interference. Use shielded twisted pair cabling from the modem to the wall jack.
Using low-quality phone wire between the phone jack and modem.
The fix: It's best to use just the phone line that was provide by the modem. If you need an extension to reach a jack, be sure it is high quality. Instead, consider using a longer ethernet cable to reach the computer (or wireless).
The fix: Make sure the ventilation ports of the modem are clear. Try mounting the modem on its side (vertically), and/or on a hard surface so that air flow is good.
Didn't work, now what?
Nothing works? Try searching the internet for ideas, or posting in the NCF DSL discussion group. If all those fail, try describing your problem to the NCF office via the Help page.