Internet Connection Issues
A faulty Internet connection may account for the game hanging up or crashing. Look for a problem icon in your systems tray. A yellow triangle or red X over your Internet connection symbol indicates a broken or poor connection. Right-click the icon and select "Troubleshoot Problems." Usually, Windows can diagnose connection problems and recommend a fix. If your connection problems persist, call your Internet service provider and request technical assistance. On the other hand, a slow connection, such as dial-up or lower tier DSL, may simply not be capable of handling the exchanges necessary to run Battle.net.
Patch Issues
Blizzard releases occasional patches for Diablo II aimed to correct bugs and improve game stability. For optimum performance on Battle.net, players should have the most recent patch applied to their game. Players without the latest version of Diablo II may experience difficulties connecting to Battle.net, including freezing and error messages. The game support pages at Blizzard.com contain information and download links for every game that the studio publishes.
Firewall Settings
If your Diablo II game works freezes when connecting to Battle.net, but works fine otherwise, then your security firewall setting may be overly restrictive. Open the control panel for your Internet security software and check the list of unblocked applications. If Diablo II is not on the list, then you will need to add it. Disabling your firewall entirely is never a good idea, as this leaves your system vulnerable to attack.
Overworked Hardware
If your computer hardware is already juggling dozens of processes, then connecting to Battle.net may send it over the edge and into a locked state. Open your task manager and ensure that only the bare essential applications are functioning in the background. If the list of applications is long and contains things you don't recognize, then your system may be laboring under spyware and adware. Do a full system scan with your security software to rid your computer of memory-hogging malware.