Hello Enulescu,
Thank you for posting your query in Microsoft Community Forum.
The issue that you are having can be due to several reasons like network adapter issues, driver issues, hardware or software issues. So, I would suggest you to try the following methods and check if the issue is reproduced.
Method 1: Follow the steps given below. ****
a) Press Windows key + X, select Device manager.
b) Right click on Network Adapters.
c) Click on Properties, and select Power management tab.
d) Uncheck “Allow the computer to turn off this device to save power”.
I also suggest you to change the power setting for network card. You can follow the article to know about power plans.
Power plans: Frequently asked questions
http://windows.microsoft.com/en-US/windows-8/power-plans-faq
Method 2: ****
Network TroubleshooterAn automated troubleshooter is a tool that can find and automatically fix some problems with your computer. These troubleshooters aren't designed to fix every problem, but they're a useful tool to try because they can often save you time and effort.
Follow the steps to run the Network 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 Network Troubleshooter in the search option.
d. Click on Network troubleshooter to run the troubleshooter.
Method 3: ****
I would suggest you to go through the links and follow the steps given below. Here are the links for your reference.
Wired and wireless network problems
Fix network adapter problems: **** ****http://windows.microsoft.com/en-US/windows-8/fix-network-adapter-problems
Why can’t I connect to a network? **** ****http://windows.microsoft.com/en-GB/windows-8/cant-connect-to-a-network
Hope this information is helpful. If the issue still persists, please post back for further assistance, we'll be glad to assist you.