I know your Virtual Private Network (VPN) used to work well, but it doesn't work now. Here I summarize some potential problems for you to consider.
Network Connectivity
Are you sure that your internet connectivity is working? A test may be needed. Try to browse a news website and read the most recent news article for sure.
Windows Update
If your Windows is currently delivering and updating patches, VPN will stop working. You have to wait for the update completes, then restart the computer to revive VPN.
Power Mode
Are you sure that the power cord has plugged to your laptop at this moment? Sometimes, the power saving mode will restrict the ability of VPN connectivity. In such case, plug the power cord no matter what before restarting your laptop.
Dialog Box
Some VPN clients may ask you to confirm something, e.g. certificates or protocols, so it popped up a dialog box for you, but you didn't notice it. Is there any hidden or minimized window? The dialog box should be confirmed in order to proceed.
SSL Connections
You may not be aware of that VPN server now accepts only Secure Sockets Layer (SSL) connections instead of Internet Protocol Security (IPsec), you should ask your colleagues or the network administrator for sure then edit the VPN profile. No restart is necessary.
In most cases, the port number of SSL is 443 by default in VPN server. Please make sure that you have all the correct information.
Two-Factor Authentication (2FA)
Maybe VPN is not broken, it just requires you to input more information to verify you, say, a One-Time Password (OTP) generated in your mobile phone or a verification code arrived in your email box. Please make sure that the OTP app is being used and works fine.
Push Notification
Some OTP app requires you to enable push notification for the application in your phone in advance. If you didn't, you may need to open it on top before using VPN.