Then you didn't do as Powdinet said.
Turning off your PC and turning it back on =/= restarting your PC. You need to specifically restart the system, or nothing will change.
To verify what eBen said above (assuming you are on windows), you can open task manager and look at the performance tab to see uptime. I didn't believe this at first either but you can clearly see when you shut down and turn back on, the uptime is still at the same value. You seriously need to click on "RESTART".
''when you shut down and turn back on, the uptime is still at the same value'' The more you know...
To explain what is happening with uptime: Since Windows 8, Window systems have a hybrid shutdown/hibernation system which is used as default when you shutdown. This system exists to speed up your computer's startup time significantly. It is possible to tell it to do a full shutdown, but it requires looking at some settings I forgot so the easiest solution is to just restart.
The GTA:SA community has been deteriorating over a long period of time. Many top runners are not currently present on the leaderboard. This issue has not been seriously addressed yet, but an attempt will be made to remedy it now.
As of this news post, the San Andreas leaderboard will function in
