Hopefully you've retained the manual. You will need any Allen wrench that came with the treadmill for maintenance. Some other tools will be needed and treadmill lubricant. Before doing any work on the belt unplug the machine. If you need a manual I have a page of treadmill manufacturers email, phone and manuals.
Lubricating your treadmill - You can get the lubricant from your manufacturer. If not, treadmills usually need silicone lube and this is available from *Amazon quite cheaply. Then you need to loosen the belt, this is where the manual is needed as it will tell you which screws to loosen. There maybe Allen wrenches that came with the treadmill.
Apply the lube between the belt and the deck. Then tighten the screws and start the treadmill at a slow speed for several minutes so the lube can spread. For more detailed instructions I have a page for lubricating your treadmill.
Treadmill belt tension - If the treadmill belt is too loose, the running surface can slow down. If it's too tight, this can cause the sticking. Turn off the machine and the belt should be able to be lifted by your fingers by around 3 inches. Too much or less is a problem. The Allen wrenches will tighten or loosen as needed and the belt should start to rotate as normal.
Treadmill belt off center - This is quite simple to fix. You need the Allen wrench's that came with the treadmill. At the back of the machine there will be a screw on either side of the roller. You tighten or loosen these to center the belt. If the belt is going to the right you tighten the left screw. And so if the belt is going to the left you do the right screw.
You need to make sure you don't loosen or tighten the screws too much otherwise that could also cause the belt to stick. Just remember the 3 inches of give in the belt when you lift it with your fingers.
What I've said above will fix your problem in 99% of the cases. If, however, the belt is still stuck then something could be wrong with the motor. This is where professional help might be needed. In this case your treadmill may still be under warranty. If so call the manufacturer and they will come and sort it for you. Outside of the warranty they may still help you, for a price.
It's a good idea to get a few quotes. Some independent repairers may offer a good deal and will be knowledgeable of a range of treadmills. I haven't used any personally so I'm reluctant to make a recommendation. But the relevant research will get you to the right people for you to do the repair.
Solving a sticky treadmill is straight forward. Take care with everything you do and make sure the treadmill is unplugged from the electrical socket. If the machine is under warranty then you may just want to call someone out in case you do something that may void the warranty. It should also cost you nothing (phone or email to check that).
The best way to prevent problems like these is to regularly maintain your treadmill. This is especially the cases where lubrication has solved the problem. That's the issue with owning a treadmill in that it needs a lot of maintenance. It's worth doing because they cost so much to begin with. Here is my page to help you for regular maintenance.