I am an Indian national residing in the UK and I got my Schengen Visa from France with an year validity from Dec'2018-Dec'2019. We were planning to visit paris on the 3rd January'19 but had to cancel it due to personal reasons. I'd like to know if I would need to enter the country within the first 3 or 6 months from the date of issue to keep it valid or does it get cancelled due to no show on the given entry date? Thanks
Turning my comment into an answer.
Life and plans change, so it is common not to fly on the original dates (especially if you acquired a visa long before your travel date). The visa is in general still valid, until the expiration date.
Note: there are two kind of validity periods (visa, not passports): the last date you can use the visa to enter and then you have normal visit period, as for US visa, or the last day of your visit, so you should enter and exit by the visa authorization/expiration times. Schengen (so your case) is this last case.
Just some points you should consider:
visa is valid for the type (reason) it was released. If it was a student visa, do not use it for tourism or work. Changing cities to visit or length of stay is ok.
you may get some extra question at immigration point, about the changes. You should have reasonable reasons. Bringing similar travel documents you delivered for visa (but for such new case) could help, especially if there is a language barrier.
some countries require a new visa if circumstances substantially changed. This is a grey area, and it is difficult to find information (it changes from country to country, and it is also subjective). I would say, if you have a change on the main points (check boxes, e.g. arrest/conviction, entering on some countries, but also name changes for example via marriage), I would ask the consular office for further guidance. Most of travel changes are not due to such changes in circumstances.
So, your case is a normal (as far I can interpret from you) change for personal reasons. You should not worry, the visa is still valid.