@Heena91,
There could be a couple of reasons why your styles are missing after a couple of days.
1. If you are working in a shared development environment, there can be times when your colleagues are overwriting your changes.
2. If you are working on a local development environment, maybe you are accidentally overwriting your client library under /apps/my-site/clientlibs/my-clientlib, when you are building and deploying code.
* Have a look at your crx/de find the client library, and take a look at the code. Determine what changes may have overwritten your client library files.
Definitely, it is standard to use the aem-project-archetype as the base for your AEM project. You can use the aem-project-archetype as a base for a POC project or production-ready website.
Any archetype version would work (making sure that the version is version compatible), https://github.com/adobe/aem-project-archetype
You can follow Arun's link for the complete list of versions that you should be using for a given AEM project via AEM environment.