Touchpad buttons wear out and/or break regularly on notebooks, and in a few various ways.
If the plastic 'button' hasn't obviously broken off or cracked, and it's just been working less and less, and is now not working, then it's sounds like the micro switch on a PC board has worn out.
Depending on the design of the notebook, this PC board will either be the mainboard itself, or a separate PC board (many models started being designed like this because of how often the switches break/wear out).
Your best starting point is to call Lenovo (or the place you purchased it from).
If you have warranty, and it's obviously not from abuse (broken plastics, chicken soup, etc.), then you should be able to get it repaired for free. If you have an active 'accidental damage coverage'-type warranty, then they should definitely fix it.
If your warranty has run out, then they should be able to help you find the nearest authorized Lenovo repair shop to help you out.
Many people who don't have warranty left, just buy a blue-tooth mouse. :)
In a couple instances I've been able to fix this issue cheap and easy, but it was because of how they failed. Here, 'easy' includes already having the notebook apart, along with having all the tools and manuals to put it back together. ;)
Usually the plastic button you click has a pointy piece of plastic sticking off the bottom of it (inside the notebook) which clicks the micro switch. In a couple of instances I've found that this pointy piece gets worn down a bit, and stops hitting the micro switch. Even though everything else looks/works fine.
I sanded the points down a little flatter, and then took a couple small, thin peices of plastic and glued them (with 5-minute epoxy) to the points, so that they were long enough to engage the micro switches.