Now the problem is solved, please add [SOLVED] to the title.
For future reference
The issue is resolved by clearing out cache. There is no miracle or fix you have done. Cache refreshes itself if you do not refresh it manually. By deleting the file you created a 404 file not found.
css is a static file that is cached, maybe by your host, maybe by external cache you have installed, maybe by opencart, maybe by your browser.
If you open your store in a browser that already has cached the file, it will still be using the cached file. You need to refresh the cache in your browser.
Each browser does it differently -
https://www.keycdn.com/support/clearing-cache
Opencart has a cog in the top right of the dashboard where you can turn it off and refresh
Your host may have a tool in your CPanel (or similar depending on your host)
Any external sources for CDN or caching resources in an extension is likely to have a button or a setting.
and as Paul has suggested - if you are using the wrong theme folder then you are not changing the right file in the first place. (style.css in the title and the file you refer to is stylesheet.css)
and JNeuhoff suggested the most likely is your browser and it has been resolved by time.
It would be wise to turn off caching in the Opencart dashboard whilst in development and learn how to clear your browser cache and check what caching systems you have.
I can not replicate this on OC4.0.2.3, my css caching system works fine