I purchased a Lenovo Ideapad S510p with Windows 8. When I brought it home I updated it to 8.1. If I let the computer go to sleep, it becomes completely unresponsive until I hard restart it (hold the power button down).

This issue may cause due to power management issues. I would suggest you to try the following methods and check if it works for you.

Method 1:
Run the Hardware and Devices troubleshooter and check.
Follow the steps to run the troubleshooter:
a. Press Windows key + C on your keyboard to show the Charms bar.
b. Type Troubleshooting and click Troubleshooting under Settings.
c. Now type Hardware Troubleshooter in the search option.
d. Click on Hardware and Devices troubleshooter to run the troubleshooter.

Method 2:
Try changing the power plan and check if it helps.
Power plans: FAQ:
http://windows.microsoft.com/en-us/windows-8/power-plans-faq

Method 3:
Try the following steps and check if the issue persists.
Follow the steps to uncheck the option “Allow the device to wake the computer”:
a. Click Start, type Device Manager in the search box and press Enter.
b. Click on Network Adapters.
c. Right click the Network Adapter that you are using and click Properties.
d. Click on Power Management.
e. Uncheck the option “Allow the device to wake the computer.”
f. Click Ok to exit the window.