It's unlikely that you've overcharged your iPhone. iPhones have built-in protections that prevent them from being overcharged. However, there are a few other things that could be causing your iPhone not to turn on.
One possibility is that your iPhone's battery is dead. If your iPhone has been sitting for a long time without being used, the battery may have drained completely. In this case, you'll need to plug your iPhone into a charger and let it charge for at least 30 minutes before trying to turn it on again.
Another possibility is that your iPhone's charging port is dirty or damaged. If the charging port is dirty, it may not be able to make a good connection with the charging cable. In this case, you can try cleaning the charging port with a toothpick and a little bit of isopropyl alcohol.
If your iPhone's charging port is damaged, you'll need to take it to a repair shop to have it fixed.
Finally, it's also possible that there is a software problem with your iPhone. If you've tried all of the above and your iPhone still won't turn on, you can try restoring your iPhone to factory settings. This will erase all of your data, so be sure to back up your iPhone before you do this.
To restore your iPhone to factory settings, follow these steps:
Connect your iPhone to a computer with iTunes installed.
Open iTunes.
Click on the "iPhone" icon in the top left corner of the iTunes window.
Click on the "Summary" tab.
Click on the "Restore" button.
Click on the "Restore" button again to confirm.
iTunes will restore your iPhone to factory settings. This will erase all of your data, so be sure to back up your iPhone before you do this.
If your iPhone still won't turn on after restoring it to factory settings, you'll need to take it to an Apple Authorized Service Provider or Apple Retail Store for further assistance.