OS X stuck, had to force shutdown

Hello

Yesterday, while using Safari, my MacBook Air got stuck. Nothing worked anymore, I couldn't even move the mouse. Other applications that were open were Mail, Text Editor and maybe some others. I had many tabs open in Safari in 3 different windows.

I left the MBA there for half an hour. It stayed as it was. Mouse stayed unmovable and screen stayed active. It didn't go into sleep either. After that I closed the lid and opened it again. Still the same. I closed it again and waited ten minutes. Then I had a black screen, when using the touchpad the mouse was visible after some seconds but the screen stayed black.

I waited again for 4 hours and nothing changed. Ten I did a force shutdown by holding the power button until the MBA was shutdown. I waited a few minutes and started the MBA. Some active applications opened automatically others, like Safari, I opened myself. In Safari I used the option to open all windows from the last session. This worked.

What could have gone wrong and why did the whole system got stuck instead of only the application?

What must I do after the force shutdown? Is there some maintenance to do?

Thanks!